Universe Size

[Red Giant]

Why is it that if the universe is around 13 billion years old, that it is thought to be 78 billion light years across? How is that possible if nothing can travel faster than light.

[Radar]

Who says the Universe is 78 billion light years across?

[RSW]

If the universe is approx 13 billion or 14 billion years old, then at light speed in both directions the Universe could be no bigger than 30 billion light years. This is just a wild guess. Maybe expansion also has something to do with it. :pipethinker:

[Astro Dave]

These figures are, at best, a good estimate. Guys, the difference in the maths is - and this is what's not stated most times this comes up - that these figures are based on Hubble's images, as we see the Universe as it was 13 odd billion years ago, and then make a size estimate. These figs quoted in the last posts are a 'guess' at what the size probably is now.

[Radar]

Hey Dave, yeah I think it would be pretty hard to measure the universe's diameter at this stage in our evolution. Hubble's images are a good guide though.

[Radar]

Here are some good Hubble Clips.

http://www.myastrospace.com/vidclip52.htm

http://www.myastrospace.com/vidclip108.htm

http://www.myastrospace.com/vidclip113.htm

[CanisMajorTom]

Those clips are cool. Really puts things in perspective. I think the ultra deep field was a 1million second exposure. :pipethinker:

[Red Giant]

One billion light years is big enough for me. :Chessy_Smile:

[Robert TG]

I'll give it a try at answering the question.

While the universe is estimated to be between 13 and 14 Billion years old but this does not restrict its size to the distance that light can travel during that period. Inflation at the instant of the big bang has make the universe much larger than the speed of light traveling during that time period.

We can talk about the size of the visible universe but the actual universe is much larger than what we can see.

Matter can not travel faster than the speed of light, but this does not restrict empty space from expanding. The universe is expanding, the empty space between matter is getting larger, without the need for the Matter to be moving at any speed.


I have often heard said that the universe is infinite but I'm convinced that is incorrect. It is BIG, so big that we will never be able to measure it's limits but it is not infinite, just way beyond our ability to measure.

[CanisMajorTom]

HI Rob,

So space is expanding between objects and objects don't even need to be moving to actually be moving away from eachother? Man that's hard to get my head around. :duh:

[Robert TG]

A lot of books give the example of placing dots on a Balloons surface. As the balloon expands the dots get further away from each other, yet the dots are not moving in relation to the surface of the balloon.

If you where living on a dot (or in a galaxy) every other dot else seems to be moving away but each of them are not moving at all in relation to their surface of the balloon.

Where it gets interesting is the fact that the universe isn't expanding exactly like a two-dimensional surface of a balloon, all points moving away from the centre. The expansion of space is in all directions / dimensions.

The other fun concept is that the expansion of the univese is accelerating. Since the force behind the acceleration is at present unknown it is called Dark Energy. Someone naming this energy has watched too many Star Wars Films. LOL.

[Radar]

Welcome to the site mate.

That is a good explanation but a baffling one at that.

So what is the general concensus of the actual speed of the expansion? At some stage, if not already, this expansion must exceed the speed of light I imagine? :hmm:

[RSW]

This subject is so deep. And the holy grail of this topic is what is powering the expansion? Dark energy? If so, where is it? Where did it come from? Can it be seen? Awesome to think that it is not only the edges of the universe that are expanding, but all of it. So we are moving away from other galaxies because the space between galaxy is expanding to? That's deep.

[Robert TG]

As an example:
If empty space is expanding at 1cm per light year distance that would still be quite an expansion over the size of the universe.

I do not know what the expansion rate is, but the rate is increasing. The above example of 1cm per light year distance is a valid picture. The question is does this 1cm expansion over this distance take a second or a year?

It makes a good question. I'll send this question into Sky and Space magazine for an answer.

How much does a light year distance of space expand in a year at the present expansion rate of the universe?

[CanisMajorTom]

How much does a light year distance of space expand in a year at the present expansion rate of the universe?

If space was expanding like this, then wouldn't this throw off red shift? For example, if we red shifted a galaxy a billion light years away, but the glalaxy was also moving because of expansion, by the time the light got to us a billion years later, the galaxy could be much further because of expansion. Which means red shift would not be very accurate. :hmm:

[Robert TG]

The expansion of the universe is the reason we have a red shift.

Yes, by the time the light reaches us from a galaxy a Billion light years away, that galaxy has been moving for a Billion years.

But when you think about it there isn't a lot of practical difference if the galaxy is a billion light years away or a billion and five light years away.

It might be important if you wanted to send a signal in the right direction, but that signal would take over a billion years to get there if we had the power required to send it.

The best we can do at this stage is observe the universe and realise that the light we are seeing comes from events in the past.

[Radar]

The expansion of the universe is the reason we have a red shift.


This is what I was thinking.

Robert, whilst we're on the topic, can you shed light (pun intended) on why Gamma Ray's can't be Red Shifted? :pipethinker:

[Robert TG]

T

...Robert, whilst we're on the topic, can you shed light (pun intended) on why Gamma Ray's can't be Red Shifted? :pipethinker:

Nope. :hmm:

I've never heard that Gamma rays can't be redshifted. They are part of the electromagnetic spectrum and as far as I know not an exception to the rule.

I have seen reports where Red shifts of Gamma Ray Bursts have been measured.

[Radar]

I have this doco that I recorded ages ago. At the moment, the way scientists measure the distance of a gamma ray, is to point a telescope at the scource of that gamma ray and redshift the light. I did a quick google. I couldn't find a quick answer to this solution but I found this following text.



Gamma-ray bursts were discovered in the late 1960s, but only recently have most astronomers agreed that a large fraction of the bursts originate in the very distant, early Universe. The bursts fade quickly at gamma-ray energies and are hard to pinpoint, making it difficult to observe a burst's optical afterglow and determine a distance, or redshift.

Redshift is a common measurement of astronomical distances. The more distant an object is from Earth, the faster it is receding due to the expansion of the Universe, and the greater its light is stretched or redshifted. This is similar to the way a siren on an ambulance appears to drop in pitch as the ambulance speeds away. Objects at high redshifts serve as probes to the early Universe, for their light has taken billions of years to reach Earth.

Yet of the thousands of gamma-ray bursts detected, fewer than ten have had an afterglow or host galaxy whose redshift could be determined with optical telescopes. This new finding by Goddard scientists has the potential of gauging the distances of many bursts from gamma-ray data alone.

Comparing the intrinsic burst luminosity (the actual brightness regardless of distance, as measured by redshifts and now, perhaps, by photon lag times) with the measured luminosity (how bright the burst appears to Earth-orbiting gamma-ray detectors) yields a distance to the source.


The afterglow of Gamma Ray Bursts is what seems to be Redshifted. I'm not sure though. :hmm:

[CanisMajorTom]

Well I did some detective work and just confused myself more.

This link doesn't actually say that gamma rays can't be red shifted, but it talks about the after glow that comes with gamma rays. And it appears that the afterglow is what is red shifted. But I'm not sure if this after glow is visible light.

http://www.nsf.gov/news/news_summ.jsp?cntn_id=104440

I'm not sure why astronomers need the afterglow

[Astro Dave]

Hi All and thanks Robert for your separate query. Look guys, you've really bitten off the big one here and I've had to go poking around for clues myself. Hope this helps;

Astronomers have been puzzling over the expansion rate of the Universe and its mass for decades. If the mass of the Universe is large enough, the expansion will eventually decrease and the Universe will then collapse in on itself.

However, if the density of matter in the Universe is less than a certain critical density, it will continue to expand for ever. The ratio of the actual density to this critical density is called Omega.

The fate of the universe is determined by the density of the universe. The mounting evidence to date favours a universe that will not collapse.

The parameter that is used to describe how fast the Universe is expanding is the Hubble constant (which tells us how fast the objects appear to be moving away from us as a function of distance).

Astronomers do not know for sure the value of the Hubble constant. In fact there are many debates going on about that, since it is a very important factor in understanding the history of the Universe. There are many estimates done with many different techniques, all of which have sizeable uncertainties associated with them.

I cannot give you a firm answer on the value of the constant, because there are so many estimates, without much to decide which is the best. The Hubble Constant is usually expressed in units of ‘kilometers per second, per Megaparsec.’

Generally, we assume the value to be between 50 and 100 km/s/Mpc (which means that for each Megaparsec (Mpc), galaxies appear to be receding at 50-100 km/s, one Mpc being equivalent to 3.2 million light years) By the way, one parsec is a unit of distance equal to about 3.2 light years.

Things become sticky when you look at developing the idea of how big the universe is… and there’s a new player in the game.

The Multiverse (or parallel universe in the singular case) scenario states that while our universe may be of finite duration, it is but one universe among many. Moreover, the physics of the multiverse may permit it to exist indefinitely.

In particular, other universes may be subject to physical laws differing from those that apply in our own universe.

[Radar]

Nice write up Dave.

So what is the universe expanding into? I guess there has to be something beyond the border of our own universe for the expansion. :hmm:

[Astro Dave]

We read that time and space themselves were born with the Big Bang, and that "nothing" existed before, and "nothing" exists beyond the universe. This of course, makes you wonder just what "nothing" meant.

If space itself is expanding, then isn't it expanding into something? I don’t buy that expanding into something jazz .. sorry, never have and never will.

Why should it have to expand into anything? Think about that. Why should the universe follow our logic and have to do something simply because we expect it to?

Think about this, “ A man said to the universe … Sir, I exist!” To which the universe replied, “That fact invokes in me no sense of obligation.” Now read it again. Don’t look at the words, read them. Understand the meaning here. “ …no sense of obligation.”

The universe doesn’t have to conform to our ideals … we have to conform to its.

I think, if anything, the universe expands into a four dimensional void. Without understanding four dimensions (which unfortunately, we can't) we cannot really appreciate exactly how it works.

Oh, I wish we didn’t have to travel this road but, here goes: M-brane Theory (supersting theory) considers that two ‘branes’ collided at a point in time which may have created our universe. It may be happening again. Is that what we’re expanding into?

By the way, Brian Greene's DVD "Elegant Universe" and the book "Fabric of the Cosmos" are great to help you down the path.

Whew … where’s the Panadol??

[Radar]

Thanks for tackling this subject Dave (it's a tough one).

I have seen the Elegant Universe and wouldn't the theory where branes are involved mean that the universe is expanding into something? Because from what I remember Branes were moving through whatever this supermassive void is.

[RSW]

Dare I ask what a Brane is? :hmm:

[Astro Dave]

G'Day Radar

Well, you see the problem (and you raised a good point) about this whole subject. No mater where you chase this issue to, and how many times you tend to analyse it, you still come back to the chicken and the egg deal.

Where I was going is saying maybe the 'current universe' (and there may very well have been many prior) is expanding into this complexive 'Brane' or higher dimensionality. (Heavy huh? Now you know why I drink so much!)

RSW, by your question you're obviously young, and no doubt fairly new to this stuff, so let's not be brutal with you. Let's remove this 'M' business (the one that makes reach for the bottle again) and tackle it's little brother .. strings.

Our physical space is observed to have only four large dimensions, and a physical theory must take this into account, but nothing prevents a theory from involving more than 4 dimensions.

In the case of string theory, consistency requires space-time to have something like10 dimensions... 9 of space and ONE of time.The conflict between observation and theory is resolved by making the unobserved dimensions compact dimensions. (errr, right...)

OK, our minds have difficulty visualizing higher dimensions because we can only move in three spatial dimensions.

Even then, we only see in 2+1 dimensions - vision in 3 dimensions would allow one to see all sides (including the inside) of an object simultaneously.

One way of dealing with this limitation is not to try to visualize higher dimensions at all, but just to think of them as extra numbers in the equations that describe the way the world works

This is my last contribution to this subject - the bottle is dry and I've now run out of time. BUT, I'd like to keep popping in to see how it develops.

(Good to see some intelligent conversation on your new forum Ray...)

[Robert TG]

Can a few forum members please check my maths and logic on this.

The question is..."How much does a light year distance of space expand in a year at the present expansion rate of the universe?"

The information Dave gave is..."Generally, we assume the value to be between 50 and 100 km/s/Mpc (which means that for each Megaparsec (Mpc), galaxies appear to be receding at 50-100 km/s, one Mpc being equivalent to 3.2 million light years) By the way, one parsec is a unit of distance equal to about 3.2 light years."

So this is the expansion rate...50 to 100 km/s/Mpc
Which works out to 15.625 to 31.25km/s/ million light year distance

60sec/minute x 60 minutes/hour x 24 hours/day x 365days per year =31,536,000 seconds per year (approx)

Therefore the expansion is 492,750,000km/year to 985,500,000km/year per million light year distance.

So ballpark, half a Billion to a Billion kilometres expansion per million light year

Light travels 299792.458 km/sec = 1,079,252,848.8 km/h
So a light year distance is approximately 9,454,000,000,000 km

So we are looking at approximately a kilometer of expansion for every 10,000 to 20,000 million kilometers of space each year.

That’s one meter for each 10 to 20 million kilometers.

So..How much does a light year distance of space expand in a year at the present expansion rate of the universe?
Answer: A light year distance expands approximately between 945km to 1,900km each year.

This seems large. Would points a just 10 billion light years apart be seperating away from each other at the speed of light? And Objects more than a 10 billion light years away would be seperating even faster. That would mean that the light leaving these objects today will never reach us.

Please check and comment.

[Radar]

:eek:

Thanks for those answer guys.

Is there anything left in that bottle Dave? Great, would you mind passing it over here please, lol.

Everything in those last two post made sense,,,,,,except for one part (yikes :eek: )



This seems large. Would points just 10 billion light years apart be seperating away from each other at the speed of light? And Objects more than a 10 billion light years away would be seperating even faster. That would mean that the light leaving these objects today will never reach us.


If the speed of light is relative to time, then no matter how fast those galaxies are moving away from us, the speed of light coming from them should still be constant. Meaning that their light (once it has made it's journey across the universe) will hit us at the same speed all other photons hit us at.

This was one of Einstein's ideas, where no matter how fast you are travelling, light will always travel away from you at the same speed, and this is possible because time is relative to the speed of light.

Does that sound right?

My turn to get the drinks :pipethinker:

[Robert TG]

Radar, you are right about the speed of light. This it is constant is the basis for relativity.

What I was trying to point out is that light being emitted from many billions of light years away will have to cross a great distance. That distance is not constant but is expanding. At some stage the distance between points will be expanding faster than the speed of light so the light can never reach across that greater distance.

For example. If a light signal is emitted from a distance of 20 Billion light years from us it is traveling at a constant speed. After a year of travel the distance it still has to go will be greater than when the signal first started.

[Radar]

Fantastic explanation Robert. I have finally abosrbed it.

Man that was deep (phew). :ta_clap5:

[Tenacious Del]

Man that was deep (phew). :ta_clap5:

My brain just exploded. :crazy:

Great thread guys.:thumbsupmate:

[Robert TG]

...So what is the universe expanding into?


This is a fun concept.

The way I see it, at the very instant of the Big Bang, the dimensions of our universe were created. This means there was no Height, Width, Length or Time before the Big Bang. Therefore there was no space for the universe to expand into. Space and Time itself was created at the Big Bang. Therefore there was no Time before the Big Bang.

Space and Time are dimensions of our universe. They are the limits of our universe.

The fact is we can still see evidence that the spatial dimensions are still expanding and we take for granted that the dimension of Time is expanding as we call it the normal flow of time, or passing time.

What is "beyond" our universe? Well, "that question" in itself is bound up by the dimensions of our universe. We can envision a "special place or time" beyond our universe only by giving that "special place" the very same four dimensions of our universe. But those four dimensions don't exist outside of our universe because those four dimensions were created at the Big Bang.

We can guess, dream or theorize all we want about "beyond the Big Bang" but the TRUTH is we don't know, and I doubt we can truly comprehend anything without our dimensions. I guess that's why many people say there is "nothing" outside of our universe.

[Radar]

Yes, it is a very open ended debate.

I feel our universe is expanding into something. The 'something' doesn't have to be like anything we have seen or known, it doesn't have to have dimensions as we know it or the same laws of physics, but beyond the borders of our universe I believe there is something we are expanding into.

[Astro Dave]

I love a debate: this is what these sort of forums are about. Good to see you are running it the right way.

I really don't want to get imbedded in the discussion again but, consider this: when you talk about 'time' and the Universe remember that there is no time in the Universe... it doesn't run by a clock and time is irrelevant there.

Time is a man-made invention! Really. Think about that. Time is a term WE coined to stop everything happening at once. Something we use to measure from one interval to another.

[Robert TG]

I love a debate...

... consider this: when you talk about 'time' and the Universe remember that there is no time in the Universe... it doesn't run by a clock and time is irrelevant there.

Time is a man-made invention! Really. Think about that. Time is a term WE coined to stop everything happening at once. Something we use to measure from one interval to another.

Is this the same philosophy as "if a tree falls in a forest, and no one is there to hear, does it make a sound"? LOL

For me Time is a dimension just as real as Height, Width and Depth. It is part of the space/time fabric of the universe. Time does not have to be running at the same speed in different parts of the Universe, and according to relativity it can be and is different as are the other three dimensions. We live in a four dimensional universe which we can see, feel and interact with.

You have mentioned string theory and "M" theory, which adds a few extra dimensions to the universe. There is no evidence for these extra dimensions and they are a creative thought to help make a math formula based string theory work. If I remember right there were SIX string theories that are valid ...so they added yet another dimension to join them together and we have "M theory". The math behind the theory is beyond me and most other people, but the theory does not have any evidence of reality.

Scientists can add unknown forces like "dark energy" to a formula to make the formula work. But "dark energy" is just a name they gave to an unknown. In the future, other factors may be discovered that will show that there is no such thing as "dark energy". Or Strings or Membranes.

But, there is a tonne of evidence to prove "Time" does exist.

According to Einstein, the speed of light is the true constant. Of course others have suggested that the speed of light may have been different at different stages of the universe.

I find it wonderful how much of the universe we humans have worked out from our gem of a planet. Even more amazing is how much we don't know.

These are exciting times, we are living in a golden age of discovery about the universe we live in.

[Radar]

These are exciting times, we are living in a golden age of discovery about the universe we live in.

I couldn't agree more. :thumbsupmate:

[Elliptical]

Thanks for the links Radar,

I showed them to my mother who was facsinated.When I mentioned that the hubble was gathering the light that was emited form systems billions of years ago and we were looking at what those systems "once" looked like, she was astounded.Totally.

Astronomy is the only field of human activity I can think of that rivals music in its endless, unknowable depth.This is one reason it is so attractive.Apart from the sheer beauty of this universe.

[Robert TG]

Music and Astronomy...the poet and the scientist...both trying to make sense of why we are here.

The word "atom" means "indivisible" in the origional Greek. It was thought matter could be divided in smaller and smaller bits until you came to the smallest bit which could not be divided. Now of course there are a whole range of things smaller than an atom. An electron, proton, neutron, six varieties of Quarks, etc...but then much much smaller than these are strings. (it's only a theory) and these strings vibrate and harmonise and make ALL that is in the universe.

So perhaps someday there is a grand unifying theory that will unite the poet and the scientist.

[Astro Dave]

Robert

Are you Carl Sagan ... reincarnated?

Just wondering, that's all.

[Radar]

Astronomy is the only field of human activity I can think of that rivals music in its endless, unknowable depth.This is one reason it is so attractive.Apart from the sheer beauty of this universe.

I agree. I think it is a very healthy way to spend time.

I imagine that it is only a matter of time before someone shows that strings can be split into something smaller.

[Duncan]

Hi guys,
Consider this.
What if Gravity waves really do exist. We already know that both light and time as we know them can be bent. But if the theory on Gravity Waves is correct then what would happen if they picked up and carried light so to speak.Then light would travel faster than its original speed.But if the speed of light is set then how can this happen? Getting confused yet?
Well i suppose the light itself would enjoy time travel i guess. After all light is only energy that we can see,and some we can't.But all light does not travel at the same speed OR DOES IT?
I haven't been there yet so i don't know.

The things we think about!!!!!!
Cheers,
Duncan:pipethinker:

[Robert TG]

"But all light does not travel at the same speed OR DOES IT?"

Light travels thru a medium (like air or glass) at a slower speed.
The general usage of the speed of light "c" is through a vacuum.

“Through any transparent or translucent medium, like glass or air, it has a lower speed than in a vacuum; the ratio of c to this slower speed is called the refractive index of the medium.”

It is interesting to note that we do not fully understand the nature of light. Our present concept of light is that it has characteristics of both a radiation wave and a particle, a photon. So the exact nature of light is not fully understood. There is a train of thought that the speed of light has changed over time, but that’s another topic.

Our present concept of the characteristics of gravity is even shakier. Over the next few years there will be some experiments carried out in space to see if there are such things as gravity waves. Like light, gravity has different characteristics, that of curved empty space and that of a particle, a graviton. There is still debate about the “speed of gravity”.

Because we think gravity is the same throughout the universe we have had to invent terms like "Dark Matter" where there is no matter to explain the additional gravity we observe in distant locations.

The term "Dark energy" is another term used to explain the expansion observations where there is no energy source observed to cause the acceleration of the universe.

Even the term "Inflation" (the early stage of the Big Bang) has come into the theory to correct what we observe (The distribution of galaxies and uniformity of the microwave background radiation) with our present understanding of Gravity.

If we discover a change in the nature of Gravity itself, we may well find that "Dark Matter", "Dark Energy", and "Inflation" are terms that will find themselves on the scape heap of science terminology, right next door to "aether".

There is so much we don’t know.

[Duncan]

Hi Rob,
Good points. But i can't wait for Dave to reply to all of this:crazy: :ahh!: :Oh_No: :hmm: :thumbsupmate:
I think i'm all the above, or getting there.
Cheers,
Duncan

[Radar]

Yeah the riddle of dark matter sure is a strange one. I think to assume that gravity is the same everywhere is wrong. We know to little to think that gravity is the same everywhere I think. We are still very much in our infancy days with physics (especially me).

[CanisMajorTom]

If the universe is expanding, surely that expansion would mean that light is slowing down? Because it has to travel further to reach the same point. :hmm:

[Radar]

If the universe is expanding, surely that expansion would mean that light is slowing down? Because it has to travel further to reach the same point. :hmm:

We're going down the rabbit hole again.

I would imagine that light speeds up with expansion, because I would think that light waves are pushed along by space time. :hmm:

[Astro Dave]

All this verbal inerchange is good for the digestive system.

This is why formus exist to consider new ideas, to console old ones, to rant and rave and have your point of view heard. And, of course ... to learn.

Support Radar in his endeavours to bring you a quality forum free of malicious intent and swollen heads. Seek out new prospective members because this is the only way it'll grow.

Radar, and his only friend in the world (his poodle dog) would love to hear from youse. (Everyone says it!)

I've got a fair bit on my plate at the moment and so can't get too involved in most ongoing discussions, especially the above. It's a cool subject but verrrry heavy!

When seeking new answers remember this; We're a 100 year old technology in a 14 billion year old Universe... we aint that smart - yet!

[Mick]

Just some random thoughts on my part OK, if the universe is expanding why can we observe galaxies colliding? Isn't the Andromeda galaxy and the Milky way eventually going to collide or merge?
Maybe the universe is being pulled apart rather than expanded like gravity in reverse?

[Robert TG]

The Universe was once thought to be a clockwork universe with everything static and very orderly. It's true nature is Chaotic. So while generally everything is getting further apart there are some galaxies that are on a collision course.

Yes the Andromeda galaxy and the Milky Way galaxy are on a “collision” course many years from now. It is interesting to note that there is so much space between the stars that these two great galaxies could pass through one another without a star directly hitting another star. The collision is in the force of gravity changing the orbits of stars in each of the galaxies as they pass. The theory is that the two galaxies will eventually merge due to gravity but in universal pin ball the outcome could be a surprise.

Mick, your idea of gravity in reverse is what some are calling dark energy. With our present understanding of gravity, something is making the universe not only expand but is accelerating the expansion. And we can't see what is the source of this energy.

In the next few years there will be a much better understanding of gravity. There are some space experiments/ gravity wave detectors to be launched as well as a huge high-speed particle accelerator coming on line. This should give scientists a better understanding of gravity and may change a lot of theories.

[Astro Dave]

The other side of the "expansion is all we see" coin is this: maybe the 'contraction is happening now, in our real time.

I'm an advocate (I hate the word believer - connotes a sheep like follower) of the 'contracting universe' theory... OK, OK, you call it the big crunch. When we look at the universe now we see it as it was billions of years ago. The light has only just reached us. That's the 'expansion' light.

Now, if the universe has by now stopped expanding and is contracting then that 'contracting' light is already on its way to us but won't be picked up for a few billion years.

[Robert TG]

I thought that over the last few years the contracting universe has been refuted along side the steady state universe in favour of the expanding universe.

You have made me very curious; could you please explain your reasoning why you still favour the contracting universe model?

[Astro Dave]

No No, Nooooo... wrong interpretation of the terminology. Nothing to do with that old theory.

I'm talking about a universe that will eventually, it is theorised, 'contract' back to where it came from . I did say some call it the 'big crunch' and this idea is gaining acceptance quickly now by many astronomers looking at how the universe may eventually end its days.

[Robert TG]

This is the old theory of the Big Crunch...

http://en.wikipedia.org/wiki/Big_Crunch

and here is a link to a number of theories.

http://en.wikipedia.org/wiki/Ultimate_fate_of_the_universe

Are you saying that the "contracting universe" is not a gravity based collapse but it is just the nature of the universe to expand and contract?

[Tenacious Del]

Now, if the universe has by now stopped expanding and is contracting then that 'contracting' light is already on its way to us but won't be picked up for a few billion years.

What about if the speed of contraction is faster than light? Then we would never see the contraction coming? My head hurts. And would the contraction be from the edges of the universe or would all parts of the universe including the space between stars and galaxies contract? I have a migrane. And what about matter? Would that contract to? I'm headbutting my keyboard. And would contraction cause gravity to become stronger? :ahh!:

[Astro Dave]

What about if the speed of contraction is faster than light? And would the contraction be from the edges of the universe ..... And would contraction cause gravity to become stronger? :ahh!:

Yes, I sympathize with your cranial discomfort there Dellio old chap. I have a little mixture which tends to cure that sort of thing ... homebrewed... errr, store bought, of course!

I firmly believe the universe will eventually fall back in on itself under a gravitational influence ... I did say 'a gravitational influence' which may be a derivitaive of what we call gravity today, but still its essentially gavitified (?). It makes sense for a universe to behave like this because that means it goes on... We're all riding a pointless expansion to nowhere otherwise!

Nature (which includes the universe) has a built in survival mechanism, you see it every day in the plant, animal, and geological world. It will resist part of it being wiped out with incredible power and divergence.

Now, you ask if the contraction would be faster than the speed of light. Well, the original 'expansion' was said to have been so I can't see any problem of it doing the same in reverse. If you really want to blow your mind read this; http://www.abc.net.au/science/news/stories/s1649585.htm

Would the contraction come from the all edges of the universe? I think so, imagine a deflating balloon.

Would gravity become stronger? Well, again I think so because of the compression of matter. Its pretty much like the formation of a black hole really, at least in general relativistic terms.

Finally, check this out - a new discovery: http://www.abc.net.au/science/news/stories/2007/1843012.htm

Now MY head hurts!!

[Robert TG]

"We're all riding a pointless expansion to nowhere otherwise!"

That sums up my view of the expanding universe.

Humans just love to have a reason for everything. When there isn't one, humans will often make one up. There is a tendance to come up with any explanation instead of simply saying, “I don’t know”.

The idea of a cycle of expansion and contraction is attractive to us as it removes the forecasted bleak ending of the expanding universe. But the universe is under no obligation to renew itself. This universe may be a one shot (bang) and it’s over universe.

Since we don’t know what caused the Big Bang in the first place, and we don’t know what is causing the acceleration of the expansion of the universe, it does follow that we would also not know what force could cause it to collapse. There is so much we don’t know.

[Robert TG]

Here are some of my basic observations of the expanding universe.

The Universe is BIG. Very BIG and all signs are that the universe is expanding and will become extremely larger than it is now.

The Universe is YOUNG. Estimates of the age of the universe range around from 13.5 Billion years old. If observations are correct and the universe is expanding then it will expand for many trillions of trillions of years. In the entire lifetime of the universe we are still in the birth stage of an extremely long life.

The Universe is EMPTY. Most of this vast universe is empty space and contains so very little matter within it. It has been said that if all the matter was spread out evenly across the entire universe that it would be a more perfect vacuum than can be created here in a laboratory on Earth. While it is not completely empty and we tend to think of the stars and galaxies in space because they are something we can see, measure and examine. These tiny islands of atomic particles are the rarity not the rule in the universe at this time. In a continual process matter is being converted to energy in stars or swallowed up into a black hole. Stephen Hawking has suggested that Black Holes will eventually evaporate. In the distant future there will be less and less matter in a greater and greater space. As the universe matures it’s true nature will be revealed as being completely empty space. If we can imagine one atom per cubic meter as nearly empty then we could say one atom per cubic billion light-year is completely empty. Due to atomic decay even that atom will cease to exist.

The Universe is COLD. At the moment the background temperature of the universe is a balmy 3 degrees above absolute zero (about 3 degrees Kelvin). This remaining heat of the Big Bang is cooling with the expansion of the universe so that the weather forecast is to expect lower temperatures ahead.

The Universe is Dark. The theory goes that the galaxies groups will become more and more separated from other galaxy groups and become isolated. Far into the future the stars will use up their fuel and new stars will no longer form due to a lack of material. The lights will go out in the universe.


When all life has ceased to be and all the matter and energy have gone, time will loose relevance, size and distance will loose relevance, the universe will loose any relevance


For the universe to stop and reverse the expansion it will need and enormous amount of energy. Gravity is not strong enough to do this.

[Robert TG]

Gee whiz, that last post stopped discussion. Sorry, I sometimes find the future so bleak that it is depressing.

[Noel Carboni]

What if the universe is exactly the same size it ever was, but the laws of physics governing what we perceive as distance are changing. Perhaps time is changing, or the speed of light.

My point is that we can't really assume anything as we are smack dab inside the experiment we're trying to observe. Without a fixed frame of reference I know I for one can't get my head around how it all works.

Indeed, what if time itself is a fully-featured dimension, quite traversable bidirectionally. It is really only because we are conscious in a reference frame in which time moves forward that we think of things as having a "beginning" and "end".

I found this illustration describing the expanding universe interesting:
http://antwrp.gsfc.nasa.gov/apod/ap060323.html

-Noel

[Radar]

Yeah this is an amazingly deep subject. We live in an exciting time though, where we can now look back in time with the big telescopes, and see the first galaxies forming. It is amazing how much has been peiced together already by scientists. I'm really looking forward to the next few years when it comes to dark energy, string theory and the universe expanding etc. I think there is going to be some amazing discoveries.

[Robert TG]

Travelling Back in Time.

As in the advanced Alien question, “Where are they?” the same question and logic could be used for a time machine that travels backward in time.

If time travel is possible then why haven’t they visited? “Where are they?”

If we say the future scientists have a “non-interference policy”, or fear that their history would change, etc, etc, then I must ask the next question.

Why bother making a time machine that could to go back in time?

If it were possible for a future human to visit the past then they would have. I think it’s obvious they haven’t, as it would be inhumane to allow the natural disasters to happen to millions without even giving a warning. But if the future time traveller is so callous as to ignore the human plight, or drop scientific hints along the way, then what would be their purpose in going back in time?

Changing the past could be extremely dangerous, so even if it could be done, no one has done it.

[Noel Carboni]

When I spoke of time being traversable forward and backward, I wasn't thinking so much of future time travelers, as perhaps another being, outside our small reality, being able to run the universe forward and backward, as we might do with the FF and Rew buttons on a remote.


Changing the past could be extremely dangerous, so even if it could be done, no one has done it.

Well, since you want to talk about human time travelers... How can you be sure no one's done it? Perhaps time travelers are all around us, helping us through difficult things all along. Nothing says they have to look different or tell us about what they're doing. It might simply be prudent to keep to one's self, for fear of being labeled mentally unstable. Or maybe we're just at an unremarkable time in a loooong history, so why would they come here?

I started a thread on this very subject on another board not long ago. Here's what I wrote:

How come we don't see time travelers from the future stopping in now and then? Is it because:

1. Humans will blow themselves off this planet before time travel is invented/discovered?
2. It is impossible to travel back through time?
3. Current history is rather uninteresting compared to what has come or will come?
4. If one traveled back through time, would one appear out in interstellar space and have to fly a long way to get to the Earth?
5. Maybe it's just REALLY expensive.

Do UFOs really contain time travelers? That might explain how they can move so strangely.

Were the multitudes of heavenly hosts singing "Glory to God in the Highest" simply groups of time travelers?

Suppose it takes a million years to invent time travel. The chance any given time traveler will come back to 2007 is a million to 1 (or more if you figure some folks might want to go back to times before human history begins). And where will they go? The world is a big place!

Surely big, important events in history will be more popular than the stuff that's happening here and now.

Maybe time travel will be possible, but expensive/difficult. Perhaps only a few very well-funded individuals - or maybe just governments - will have the resources. Research missions may be few and far between.

Maybe it will be 1 and 3 above, and some new species will rise to the top of the food chain. For the sake of argument, assume they're the distant descendants of modern day cockroaches. Will they want to come back to see us humans? Hardly.

-Noel

[Radar]

Both good arguments guys.

Maybe there are no time travellers visiting us yet because the current time we are in is the latest possible time to be in, so time travel has yet to be invented? There has to be a first version of 2007 in order to get to 3007 where time travel is created.

Also if you do travel back in time, how does the time machine know where in the universe to place you? The Solar system would have moved trillions of Kms by the time that time travel is invented. So not only would it need to move you back in time, but also it would need to move you around the galaxy to get you back into the spot that coincides with where Earth was then.

[Mick]

Surely big, important events in history will be more popular than the stuff that's happening here and now.-Noel

I don't see how here and now applies to a timetravler. If time is forward, reverse, 2007 would be ancient history way into the future or is that the past. Sometimes the little, small events are the beginning of big changes for the future :hmm:

[Radar]

I don't see how here and now applies to a timetravler. If time is forward, reverse, 2007 would be ancient history way into the future or is that the past. Sometimes the little, small events are the beginning of big changes for the future :hmm:

I agree with you and Noel. Here and now wouldn't apply or appeal to a time traveller. Major events in human history sure would though. So the best place to look for time travellers is when something amazing or major is happening around the place.

Disasters like 911, or good things like humans first landing on the moon may attract time travel tourists. So next time something big happens, check the crowd out and hone in on whoever looks out of place.

302

[Astro Dave]

These ideas/theories/comments being bandied around here are probably one of, if not THE most talked about subject on 'General Discussion' boards on astronomy forums.

The idea of a 'Time Machine' has always captured our imaginations. To be able to move freely back and forth through time with gay abandon (watch that one) is something we'd all like to do of course - to "go back and do it all again" appeals to any intelligent organism with delicately finite lifespans...like us.

Unfortunately we tend to dwell too much on the fantasy of the idea itself rather than the practicalities, or the possibilities.

For my point of view the word "time travel", although a romantic enough idea, is a contradiction in terms. To travel forward implies there's a 'forward' to travel to... a 'future' if you must.

The sad fact folks is there is no future - it hasn't happened yet! Read again , it hasn't happened yet.

Comments like "the future is pre-ordained" are plain crap. The future - your future- is determined second by second from what you do now, where you go, and the decisions you make ... second by second.

To reason that there are no time machines or time travel is impossible because no-one has yet come back from the future is a ridiculous way to look at it. It assumes there is a future out there just waiting for us to run into. They haven't come back because there is no future there to come back from. There will be someday, BUT it will be the present for those in it.

Time moves forward for us. We can, according to relativistic theory, modify it personally ie/ slow it down) but we can't change it's outcomes.

Time travel into 'the past' is also a romantic idea. Sure we'd all like to go back and right that wrong, turn left at that intersection instead of rightt, or just reconnect with Mom & Dad (see the episode "Walking Distance"- Twilight Zone with Rod Serling) but we forget about the realities of all this: we call it causality ... cause and effect.

Time travel into the future could only, in reality, be accomplished for an individual experiencing speeds approaching that of light - or from his presence close to or in a strong gravity field.

He hasn't actually "travelled forward in time" - time for him has been arrested somewhat due to his speed (or gravitational situation), he has simply slowed down enabling him to sort of live longer in relation to the universe around him, which has incidentally moved forward unaware of his presence.

The paradoxes of reverse time travel are not only enormous, I think they would be insurmountable. Somewhere in physics will be a 'rule' that prevents this happening.

IF one day time machines could be made can you imagine thousands of time machines roaming back throughout the past centuries all with their individual riders making constant changes to history? What a shemozzle!!

What about the future in this scenario? Yeah, just as I thought, if any argument can be put up against time travel this is it.

We exist in this time, now! There is NO future. There is a past BUT it is gone.

Maybe we can one day do the time travel thing but move into different 'dimensions' or 'alternate universes' instead of mucking up our own. That, as they say, is a horse of a different colour.

My point of view - good discussion.

[Robert TG]

In a sense we are all time travellers. We are all born and live our life of some years.

So what has the experience taught us?

My life has been lived in the NOW. At no time in my life did I think that I was living in the past or in the future. For me it has always been “now”. I have heard the term “Forever Now” used to describe this reality experience.

We say we can see Forward Time flow from the past to the future. What we actually are seeing is cause and effect. An egg is dropped, we see it fall and smash. We experience all of this in the “now”, and then we remember the event as the past.

To travel back in time is to reverse cause and effect. We would see a smashed egg pull together and rise up into the air and into our hand. Having the effect first then the cause is Reverse Time. I also believe the physics in this scenario is insurmountable.

The reverse time effect can be demonstrated by placing the frames of a film in reverse order. When we view this reversal it would be in the “now”.

The true nature of time needs to be understood. I’m not sure anyone has the definite complete answer to this dimension. Time is a real dimension of our universe, as everything needs time to exist just as they need the three special dimensions to exist. Science fiction does not need to portray time's true nature of anything but can present some very entertaining ideas shrouded in mystery. i.e. wormholes, blackholes, etc, etc all having unbelievable amazing properties, but the properties are fiction.

Relativity allows for the dimensions of space-time to be distorted and different observers will observe events differently from their perspective, this is strange enough for me without the need for added fiction.

I'd agree with Noel's posibility number "2. It is impossible to travel back through time"

[Noel Carboni]

It is clear we don't understand all there is to understand about the nature of time. For that reason I would certainly hesitate to make any definitive statement.

The very fact that we can predict some things indicates we have an inkling of how it works, though. For example, we can predict the orbits of objects, given a sufficiently accurate measurement of mass, motion, and nearby mass.

Now here's what'll bake one's noodle... Say we do predict the orbit of something, and if that object truly does what we've predicted, then one could say what the object did was inevitable, which seems true enough given the laws of physics as we know them. This implies the present DOES set the future, much as the past has set the present, as long as one takes enough data into account. This begins to imply the nature of all things is inevitable, and that the universe is traveling on a set path from past into future. Frankly, if it weren't, everything would be in chaos. So... For a hypothetical consciousness that exists outside of time as we know it - and why should such a thing be difficult to imagine - it might be possible to traverse time as simply as we move from point a to point b in the first 3 dimensions. It may require some energy is all.

Note that I didn't say we are likely to elevate anyone from around here to that level of consciousness any time soon.

And nothing here has implied that the past or future can be changed.

Just more food for thought.

-Noel

[Astro Dave]

"What time is it??" What is the true, definitive, correct answer to this question?

[Robert TG]

"What time is it??"
It's Now, of course.

[Radar]

Good discussion guys.

It would be good to listen into a physicist at a Uni talking about this.
The only lecture I have ever attended about Time was from my girlfriend when we were supposed to be going out once and I was late. :duh:

[Astro Dave]

I think, Robert (and these are only musings from long dormant neurons) that before an answer could be given you'd need to explore the question. Something along these lines:

Q. "What Time is it?'
A. "Where are you and how fast are you moving?"

Explanation:
Where are you(ie/strong gravity field?) and how fast are you moving(relative to light speed)

[Noel Carboni]

I was thinking along the lines of one of these two answers:

1. 5:26 pm.

or

2. Relative to what?

:)

-Noel

[Astro Dave]

Hi Noel. Gidday Cobber! (They really do talk like this around here)

Radar has kangaroos roaming throughout the main street - he's a bit of a bohemian (and I don't believe they're real smokes either)

Look, this time thing is really only relevant to you and anyone moving with you at the same rate and in the same spatial reference isn't it? I mean, it has no meaning to some bod moving past you at 1/3 C. Your clocks can't agree.

.. BUT you both have the right time - for you!