# Where is the future if everywhere is the past?

1. Sep 23, 2014

### iDimension

I just wanted to check as I'm sure I know the answer but if someone can verify it for me. Everywhere we look out into space is the past relative to us and likewise any life on some other planet would consider us to be the past.

Is this simply because it's the same thing as there is no centre of the universe because everywhere is the centre relative to its surroundings? So as our observable universe becomes larger, the light from the newly revealed space will become visible to us providing us with an even deeper look into the past? Is this correct?

If so then I'm confused because wasn't there a recent WMAP or some other satellite that can see back to $10^{-7}$s or something? Sorry for the guess but I know it's something like that.

What am I misunderstanding?

Thanks.

2. Sep 23, 2014

### ShayanJ

The fact that we see the past when we look at distant stars is not something related to the universe having a centre or not!
Even when we look at the sun, we see it as it was 8 minutes ago. When you look at your keyboard, you see it as it was in the past, but only a very very very very......very tiny time ago! Its only a consequence of the finite speed of light, it takes time for light to go from a point to another but because its so fast, the distance between those two points should be very very very large for an observable effect to exist.
And WMAP is not that kind of a telescope and the fact that it can "see" back to $10^{-7}$ after big bang, is not because of such things but something very different.

3. Sep 23, 2014

### Doug Huffman

The future is not yet, at least according to Karl Popper and Lee Smolin, because it is not pre-determined. From Popper's argument, it is for human freewill. From Smolin's argument, well it's less straight forward.

4. Sep 23, 2014

### iDimension

Both of you have not really answered the question. @Shyan I already know everything you mentioned.

5. Sep 23, 2014

### bapowell

What you write is correct, iDimension. But what exactly about WMAP confuses you? Why does it not fit with the cosmological principle and the idea that as we look out into space we look back in time?

The CMB that WMAP measures more or less uniformly fills the observable universe. The CMB photons that we receive today here on Earth, and the CMB photons that are measured today by some alien race in Andromeda, took the same amount of time to make their respective journeys. In both cases these photons make up the oldest light observable.

6. Sep 23, 2014

### iDimension

I'm not sure if it was the WMAP satellite that looked back in time the furthest but it was a satellite of some sort, basically if we can already see back to $10^{-7}$s then as the observable universe expands we'll be able to see further into the past but the past is finite which means what happens when we can see back to $t=0$ but still more observable universe is coming into view?

Suddenly we are seeing further into the past than time has existed, do you see what I mean?

7. Sep 23, 2014

### bapowell

I see your confusion. Hopefully this helps.

At any given time, we receive light on earth coming from many distant sources. Suppose that we receive light from star A and star B at the same time, but that star A is closer than star B. Then, we obviously conclude that star B emitted its light earlier: star B emitted its light when the universe was younger. Now, say that we again observe these two stars several years from now, so that some expansion has taken place and that they are now farther away. These stars have not moved into the past! In fact, the light we now observe from both A and B was emitted after the light we observed several years ago, despite the fact that it's coming from more distant reaches.

At any given time, we in principle have a line of sight all the way back to the big bang: from nearby stars, to galaxies, to more distant quasars, to the CMB. We can't see beyond the CMB because the universe was not transparent to light back then, but if we could -- if light could somehow escape the dense plasma of the very early universe -- we would be able to see back to the big bang. This is all true regardless of expansion, because I'm talking about a single instant in time. Now, allow the universe to expand. The only effect is that these milestones in time: the big bang, the creation of the CMB, the formation of the first starts, etc., get farther away in space. Sure, it takes their light longer in time to reach us as they expand away from us, but the instant in time in which they originally occurred (t = 0 for the big bang, t = 400,000 years for the CMB) are fixed. The universe is simply getting older.

Last edited: Sep 23, 2014
8. Sep 23, 2014

### Staff: Mentor

No, because we're moving into the future as the observable universe becomes larger. The light reaching us from the furthest part of the universe we can see is getting older, yes, but so is the universe itself.

For a simple example of how this works, suppose the universe were not expanding (so that we can equate distances with light travel times, for simplicity). Suppose right now the observable universe goes out to ten billion light-years from us--that means (in this simple model) the light just reaching us now from the edge of the observable universe was emitted ten billion years ago. Call that time $t = 0$, so that we, right now, are at time $t = 10 \times 10^9$.

A billion years from now, the observable universe would go out to eleven billion light-years from us (remember, in this simple model the universe is not expanding). So the light reaching us then from the edge of the observable universe would have been emitted eleven billion years before we receive it. But that means, since we will be at $t = 11 \times 10^9$ a billion years from now, that the light reaching us then from the edge of the observable universe will have been emitted at $t = 0$--exactly the same time as the light reaching us now, from what is now the edge of the observable universe.

In a more realistic model, where the universe is expanding, the relationship between the distance to the edge of the observable universe and the travel time of light just reaching us from the edge of the observable universe is more complicated. But the conclusion of the above is still the same: light reaching us at a given instant, from the edge of the observable universe at that instant, is always emitted at the same cosmological time (what I called $t = 0$ above). So it always provides us a look back to the same point in the past.

9. Sep 23, 2014

### iDimension

10. Sep 23, 2014

### Chronos

Bear in mind that photons emitted from the remote universe are also time dilated by the factor t0 = t1 / (1+z), where t0 = time at the source, t1 = time on earth and z = redshift [as measured on earth]. This means after another billion years on earth, the CMB photons we observe then will have been emitted less than a million years after the ones we currently observe. It also means that a signal emitted immediately after the big bang itself [where z approaches infinity] is essentially almost as old as the big bang itself. This time dilation thing [predicted by general relativity] is why, in theory, we can look back an almost unlimited amount of time into the past with the proper instruments. The problem with the CMB is it is opaque to all forms of EM radiation. This impenetrable wall formed during the big bang and did not 'lose' it opacity until about 380,000 years thereafter. Neutrinos and gravity waves are the only things we know that would be detectable from when the universe was younger than the CMB. This should help explain why gravity wave and neutrino astronomy is such a big deal in cosmology.

11. Sep 23, 2014

### Staff: Mentor

This is because of the expansion of the universe, right? (That is, the redshift of about 1000 of the CMB photons, due to the expansion of the universe by that factor, corresponds to a factor of about 1000 ratio of the time elapsed between us receiving two photons and the time elapsed between them being emitted.)

12. Sep 23, 2014

### Chronos

Yes, absolutely. It was convincingly shown as a consequence of the SNLS study re: http://arxiv.org/abs/0804.3595, Time Dilation in Type Ia Supernova Spectra at High Redshift.