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The Universe’s past and our past


pmourad
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I have been thinking about the “past” in the universe and what the past mean to us, and I came across a thought that I would like to share and hopefully get some feedback and discussion.

When reading books on astrophysics, they all mention that the universe is expanding, and probably has been expanding for some time now. That means if there is a star close to us today, tomorrow that star will be farther away, because the universe is expanding. Let’s imagine that from earth in the year 2019 we can see the Star “X”. Then, in the year 3000 the universe has continue to expand and therefore Star “X” is farther away from us but we can still see it. 10.000 years later, as the universe has continued to expand, Star “X” is so far away from earth that we cannot see it anymore, but we know it is there. We would talk about the Star “X” as something that happened in “the past”; but it can’t really be in the past because it still exists, therefore the Star “X” still IS, in the present.  

The difference I see with our “humanly” past, is that our past is over, it’s a moment that happens and ends, doesn’t continue. We cannot see it, hear it, measure it; we can only remember it. Our past has an end. But is there a past in the universe? The expansion we know of today, if it is a result of the big bang, doesn’t it mean that it is still happening and therefore it is not in the past? 

I drew a little sketch of my example because I think that it might help to explain myself :)

 

 

48434072-5F04-4791-94E9-8636879A36C5.jpeg

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Everything you see is in the past.
Light, and any information, moves at a finite speed, such that an object 300,000 Km away is 'seen' as it was 1 second ago.
But since we can never see it as it is now ( there is no universal 'now' ), the reality we experience ( even effects on us ) are governed by this speed of light limitation.

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1 hour ago, pmourad said:

I have been thinking about the “past” in the universe and what the past mean to us, and I came across a thought that I would like to share and hopefully get some feedback and discussion.

When reading books on astrophysics, they all mention that the universe is expanding, and probably has been expanding for some time now. That means if there is a star close to us today, tomorrow that star will be farther away, because the universe is expanding.

No. The universe is expanding only over the larger scales. Over small scales such as our solar neighbourhood, our galaxy, our local group of galaxies, and even the cluster we are apart of, are "decoupled" from that expansion rate due to gravity.

You are correct though that due to the expansion of space over large scales, galaxies near the observational horizon, will eventually be unable to be seen.   And of course every time we look at the stars, we are seeing them as they were.eg; Alpha Centauri system...4.5 years ago, Andromeda galaxy, 2 million years ago.   

Edited by beecee
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6 hours ago, pmourad said:

When reading books on astrophysics, they all mention that the universe is expanding, and probably has been expanding for some time now.

It has always been expanding! (Well, since the earliest times we can observe. We don't know what happened before that. Maybe it collapsed from an earlier universe.)

6 hours ago, pmourad said:

That means if there is a star close to us today, tomorrow that star will be farther away, because the universe is expanding.

Actually, expansion doesn't affect the stars near us (in our galaxy). It doesn't even cause nearby galaxies - the Andromeda galaxy is heading towards ours and they will collide in a few billion years.

Quote

That means if there is a star close to us today, tomorrow that star will be farther away, because the universe is expanding. Let’s imagine that from earth in the year 2019 we can see the Star “X”. Then, in the year 3000 the universe has continue to expand and therefore Star “X” is farther away from us but we can still see it. 10.000 years later, as the universe has continued to expand, Star “X” is so far away from earth that we cannot see it anymore, but we know it is there. We would talk about the Star “X” as something that happened in “the past”; but it can’t really be in the past because it still exists, therefore the Star “X” still IS, in the present. 

There are galaxies so far away we cannot see them (so we just assume they are there).

But I don't really understand your point about the past. When you look at anything in the sky, you are seeing it as it was in the past - the further away it is, the more in the past we see it. (This is because of the time it takes light to reach us.)

For example, the star Betelgeuse is likely to explode into a supernova some time soon. It could even have already happened but we won't see it until the light reaches us, which will take 642 years. So when we do see it explode, we will be seeing what happened 642 years ago!

 

38 minutes ago, michel123456 said:

Do we actually observe that? Galaxies fading away?

No. The timescales are far too long.

And, actually, I think that because of accelerating expansion (so-called "dark energy") the cosmological horizon is receding and so more galaxies will become visible. (But I could be mistaken about that).

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2 hours ago, Strange said:

And, actually, I think that because of accelerating expansion (so-called "dark energy") the cosmological horizon is receding and so more galaxies will become visible. (But I could be mistaken about that).

I remember another thread about L. Krauss theoretical prediction that future beings observing the Universe would observe only their own galaxy & would conclude that the whole Universe is made of only one single galaxy (their own).

Here

 

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1 hour ago, michel123456 said:

I remember another thread about L. Krauss theoretical prediction that future beings observing the Universe would observe only their own galaxy & would conclude that the whole Universe is made of only one single galaxy (their own).

Yes, you are right. I have it the wrong way round. Accelerating expansion means that galaxies will disappear over the cosmological horizon. 

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Quote

But I don't really understand your point about the past. When you look at anything in the sky, you are seeing it as it was in the past - the further away it is, the more in the past we see it. (This is because of the time it takes light to reach us.)

My point is that if what we are seeing is the past, then it's a different conception of "past" as we, humans, know it. Our own past, what we did yesterday, las week, last year, happened then and therefore we cannot see it in the present. It was an action that took place in a moment of our lives and ended, doesn't continue. Taking that into account, if today (the present) we look at the sky and are able to see the past, then there are "two pasts": our humanly past (the one that ends) and a universe's past, the one that we can still see.  I hope I was able to explain myself (I find it hard myself to explain my question) :D

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4 minutes ago, pmourad said:

My point is that if what we are seeing is the past, then it's a different conception of "past" as we, humans, know it. Our own past, what we did yesterday, las week, last year, happened then and therefore we cannot see it in the present. It was an action that took place in a moment of our lives and ended, doesn't continue. Taking that into account, if today (the present) we look at the sky and are able to see the past, then there are "two pasts": our humanly past (the one that ends) and a universe's past, the one that we can still see.  I hope I was able to explain myself (I find it hard myself to explain my question) :D

You are right that the concepts of "now" and "past" become complicated when we have to take the delays caused by the speed of light into account.

But it isn't that much different from things that are more familiar. If you hear distant thunder (but don't see the lightning) you know that the lightning happened in the past and you were only "now" becoming aware of it.

So when we look at the sky (stars and planets) as they are "now" were are seeing them as they were at various times in the past (depending how far away they are). You could create a map of the stars showing where they "really" are now but it wouldn't be terribly useful and, given the distances and speeds involved, wouldn't look much different. (Unless Betelgeuse really has exploded.)

But, it gets even more complicated than that. Because we know from the theory of relativity that different observers see time passing at different rates (because of their relative speed or the effects of gravity) and so not everyone agrees what "now" is. What might be in the past for some people might be in the future for others.

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4 hours ago, pmourad said:

My point is that if what we are seeing is the past, then it's a different conception of "past" as we, humans, know it. Our own past, what we did yesterday, las week, last year, happened then and therefore we cannot see it in the present. It was an action that took place in a moment of our lives and ended, doesn't continue. Taking that into account, if today (the present) we look at the sky and are able to see the past, then there are "two pasts": our humanly past (the one that ends) and a universe's past, the one that we can still see.  I hope I was able to explain myself (I find it hard myself to explain my question) :D

Here below a Spacetime diagram. In this diagram, time runs from bottom to up. Space is reduced to 1 dimension. The sheet of paper is 2 dimensional and represents Spacetime.

You are at the red point as an observer at T=0

Conventionally, the Speed Of light is represented by a diagonal at 45 degrees. Since time goes by convention from bottom to up, the past is on the bottom of the diagram. The future is above.

events-b.thumb.jpg.1295bf246d791569abd1dff37ac9589f.jpg

The vertical line is your "life line". You don't move from your chair.

Event 1 is yourself in the past (say 2 years ago).

Event 2 is something that happened far away from you, you saw this event happening 1  year ago when you were at point a (this event happened 1 LY away from you).

All the events from which you can directly observe through information traveling at SOL are upon the blue lines: it is the image you get from the universe looking at the stars.

The set of events in the green zone are what we call " the past".

All the rest of the diagram is not observable. It will be observable in the future, as time goes by (that is to say: as the red dot travels up in the diagram).

In this representation, YOU are traveling in time, time itself is a simple background (like space)

And please take note that event 2, which belongs to the past, is not currently directly observable.

Similarly, your own past (event 1) is not directly observable by you.

If anyone detects an error please inform me.

Edited by michel123456
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7 hours ago, pmourad said:

My point is that if what we are seeing is the past, then it's a different conception of "past" as we, humans, know it. Our own past, what we did yesterday, las week, last year, happened then and therefore we cannot see it in the present. It was an action that took place in a moment of our lives and ended, doesn't continue. Taking that into account, if today (the present) we look at the sky and are able to see the past, then there are "two pasts": our humanly past (the one that ends) and a universe's past, the one that we can still see.  I hope I was able to explain myself (I find it hard myself to explain my question) :D

The finite speed of light ensures that there is no universal "now". Any "now" is defined when photons reach our eyes. The further away a photon/light originates from, the further back in time we are looking. 

 

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