Question regarding ascertaining the age of the universe

Factual Questions
1

First things first - I am not that knowledgable is cosmological things. I will probably make some wrong boneheaded statements and show my ignorance but I hope the spirit of my question comes through. Anyway…

I was listening to this great podcast about the universe. Specicfically how we determined it’s age and beginning. (I can’t remember the exact episode but it was by this guy - http://www.learnoutloud.com/Podcast-Directory/Science/Astronomy/Astronomy-162--Stars-Galaxies-and-the-Universe-Podcast/22804)

As I understand it based on other theories and information, Hubble was able to determine through red shift observations that the universe was expanding. I realize that based on that other factors were brought in to help cement and more precisely measure it’s age, such as background radiation, etc. but from what I can determine it seems that Hubble really got the ball rolling with direct observation.

There are several theories as to the end of the universe, one being that it is a closed system that will one day result in a Big Crunch.

Based on the above my question is this, if we had came into being in a time frame that coincided with the extent of the expansion (in other words right as it slowed to a stop before it started contracting), would we have been able to figure out that there even was a Big Bang or how old the universe was without the red shift phenomenon?

Also, what if we had come into being as it was ‘crunching’, would blue shifts tell us that? Could we have surmised a Big Bang from that and determined the age of the universe?

2

These days models which include a contracting phase and ultimately the big crunch are no longer standard cosmologies as they don’t match up to observation, but to answer your questions first it is best to understand a little bit about expansion and how it relates to redshift (skip to the last three paragraphs if you just want the answers though):

The expansion of the Universe can be tracked by something called the ‘scale factor’, this tracks the relative difference in distances at various stages in the Universe’s history. By definition the present scale factor is 1; in the future where expansion has caused a faraway galaxy to be twice as faraway from us as it is now the scale factor would be 2; in the past where the same faraway galaxy was half the distance it is now away the scale factor would be 0.5; etc.

Hubble shift (i.e. redshift due to expansion) doesn’t depend on the current rate of expansion/contraction, where the rate of expansion/contraction can be defined as how quickly the scale factor changes, instead it depends on:

(scale factor when the light is observed)÷(scale factor when the light was emitted).

If you like this because Hubble shift is due to the wavelength of light being stretched with the expansion of the Universe. Unfortunately Hubble shift has often been explained popularly as Doppler shift due to recession velocity, but this is misleading.

The upshot of this is that in a Universe that starts in an expanding phase and then goes to a final contracting phase, at the point when it turns around, objects will still appear to be increasing redshifted the further away they are. If you like this is because the scale of the Universe was smaller when the light was emitted when it was received.

In the very final stages of such a Universe, just before the big crunch, most galaxies would appear to be blue-shifted. However due to the earlier expanding phase the relationship of Hubble shift to distance would be: at first that the further away a galaxy is, the more blueshifted it would appear, up until a certain very far away point where galaxies would become less blueshifted with distance. Slightly further away still and galaxies would not be Hubble shifted at all, and the very furthest observable galaxies would become increasingly redshifted (this would be right on the edge of the observable Universe).

So evidence of expansion would still exist in the Universe right up until the big crunch. However expansion would be harder to detect. For example at the turnaround point the relationship of redshift to distance would be very flat for nearer galaxies, meaning more powerful telescopes and more sensitive instruments would be needed to detect the previous expanding phase.

3

As I realized I didn’t properly answer your question:

In a Universe that starts with a big bang and ends in a big crunch, the big bang is the past point where the scale factor goes to zero and the big crunch is the future point where the scale factor goes to zero. As Hubble shift is related to the evolution of the scale factor and distance is related to time and he evolution of the scale factor, precisely determining the relationship between distance and Hubble shift will always allow you to determine the evolution of the scale factor and hence the age of the Universe.

In practice though calculating the age of the Universe is more complicated and is done by matching the data to the best model.

4

Thanks Af,

I appreciate that detailed response. I’m not sure I completely understand all of it, especially how the scale factor works, but it’s fascinating and its prompted me to study up on it more.

5

It struck me that it would be interesting to visualise the universe as one where the universe remained the same size, but everything within it was shrinking. Imagine a room full of people, and they all start to shrink. And shrink and shrink and shrink. Even a person an arm-span’s distance away from you would eventually appear to be receding from you faster than you could run.

But the speed of sound in the room would remain the same - as the air has not changed. But the apparent wavelength of the sound as you hear it would drop. So if someone shouted at you from the other side of the room, by the time the sound reached you, you would have shrunk even more and the wavelength as you perceive it would now be much longer than when the person shouted.
If you knew how fast everyone was shrinking you could use that fact to work out how far away the other person was when they shouted at you in terms of your current size.

Not sure how far one can push the analogue, but it would be fun to see.

But somehow I found it easier for (at least my) mind to actually visualise the idea. Visualising space stretching is much harder. So the thought might have some utility.

6

The scale factor itself is a fairly straightforward concept that can be applied to simpler situations. For example lets say you have an photo you want to blow-up to twice its linear dimensions: the scale factor in such a scaling is 2. Lets say you have the same photo and want to shrink it to half its linear dimensions: the scale factor of such a scaling 0.5.

What is be scaled as the Universe expands is the Universe on a large scale, i.e. the scale of non-gravitationally bound galaxies. E.g. if a galaxy is currently 250 million ly away, at a future point when the scale factor is 2 it will be 500 million ly away, as the linear dimensions of the Universe on a large scale have increased by a factor of 2.

It’s worth noting though that on a smaller scale (i.e. galactic cluster and downwards) the linear dimensions are not affected by expansion as objects are bound together by forces whose strength does not change with the scale factor. Indeed if such forces did change linearly with the scale factor there would be no way to tell that the Universe was expanding.

This can still be a little confusing as in Newtonian physics systems can’t scale isotropically (the same in all directions) and smoothly. However in general relativity spacetime curvature can allow systems to change scale with time.

Francis Vaughan makes a good point: we could equally say that rather than expanding, the linear dimensions of the large scale Universe were constant, but that structures at the galactic cluster scale and below are shrinking. Both views are equally valid as we can only compare the small scale against the large scale and say one is shrinking or one is expanding. However there are good reasons for choosing the expanding Universe paradigm, for example if we say objects are shrinking then we would have to say the strength of the fundamental forces are increasing with time.

7

The problem I’m having with scale factor is not with the concept of it but how we can apply it to the age/size of the universe. Unlike your photo analogy, the only snapshot we have of the universe is the one we are living in. The size of the universe is 1. Halfway to this stage it can be labeled as .5. Got it. But how do we determine how long ago that was? How do we determine from that information the speed of the expansion?

8

Why does the universe have to have an age?

Or why does there need to be a beginning to the universe?

Is it because humans are “born”, so therefore everything else MUST be that way too?

How about this: The universe goes on and on for “infinity”. There is no “end” to it! Also there was no "beginning and there will not be an “end” to it. The universe was always here and will always be here!

It is just that humans can’t seem to grasp this concept…
(Note astronomers have a LONG history of being WRONG!)

9

Not true. Because the speed of light is finite, we can see earlier eras of the Universe by looking at more distant objects. In fact, there’s no one single time at which we can see an entire snapshot of the Universe: All we can see is many partial snapshots at many different ages.

The Universe doesn’t “have to have” an age, and astronomers are quite capable of grasping the concept of a Universe that always existed. In fact, until some time in the past century, this was the dominant scientific view of the Universe. The reason we now consider it to have a finite age is not “because that’s just the way it ought to be”; it’s because that’s what the observational evidence shows us.

(but I seem to recall explaining this to you before, and it didn’t seem to stick then, so I’m not sure it will now, either)

10

An important piece you might be missing is that redshift isn’t the only way we have of telling how far away something is. There are so-called “standard candles”, though it does get hairy fast. A classic example of a cosmological standard candle are Type Ia supernovae which, due to their explosion mechanism, produce a consistent peak brightness. Thus you can figure out how far away one is based on its perceived brightness at earth by figuring for 1/r[sup]2[/sup] losses plus any attenuation due to foreground material. If you also measure the supernova’s redshift using spectral lines, and you note that the speed of light is constant, then you have everything you need to pinpoint how far away the object was when it emitted the light, how long ago it emitted the light, and what the scale factor was when it emitted the light.

If you do this for enough objects over a spread of distances, you can build up the relationship between scale factor and distance and, also, scale factor and time. (Turning this history into a prediction for the future requires a bit more input.)

Given that we have such a relationship in-hand today, one typically uses scale factor as a shorthand for distance or time, cast in terms of z:

z = (scale today) / (scale back then) - 1 ,

so that “here/today” is at z=0, with larger z values corresponding to farther away / further into the past. This quantity z is the “redshift” you see in astro/cosmo discussion, for example in this list of gamma ray bursts.

11

There will indeed be a time in the future at which the evidence of the big bang will be lost to us, but it has nothing to do with the universe’s expansion slowing down or stopping. In fact, it’s just the opposite. The best evidence we have now says that the universe is expanding, and that the rate of expansion is increasing. That’s due to ‘dark energy’, whatever that is. But because of this, there will come a time when the things most distant from us will be outside our ‘light bubble’. Eventually, the only things in the sky we will be able to see will be the set of galaxies we are gravitationally bound to. Everything else will be so far away it will be impossible to observe.

Now, we talking about a long, long time from now. Hundreds of billions of years. But if a new intelligent species sprung up then, they would have no evidence whatsoever of the Big Bang, and would probably believe that the universe consists of the local group of galaxies and nothing else.

12

Stuff like that is overwhelming to consider. What a frustrating time it will be for scientists (I’m assuming we won’t be able to pass our knowledge along to them). But more importantly it makes me wonder what we may be missing because of some similar limitations.

Chronos - thanks for that perspective on our current “snapshot”. I feel silly for not realizing it myself.

Everyone else - thank you for helping me to understand this difficult (for me) subject.

13

At the same time it’s quite astonishing that we can have any inkling at all of the origins and nature of our universe, however imperfect, given the scale of time and size to which we are constrained.