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Scientists say that out universe is expanding faster and faster. This was detected via the red-shift effect. Farther galaxies are moving from us faster than those that are more closely.

But how can scientists tell this is happening now? They can see what WAS in the past. As I understood red-shift shows to us what was 10+ million years ago if the distance between us and the other galaxy is 10+ million light-years.

Closer galaxies move away more slowly. But this WAS, lets say, 5 million years ago if the distance between us and that galaxy is 5 million light-years.

The distance between us and Andromeda is 2.5 million light-years and it is moving towards us (or we are moving towards it).

As I understand the speed of expansion gets slower and slower in time. And Andromeda shows that 2.5 million years ago the direction switched to compression. Why do scientists say that expansion becomes faster and faster with time?

Qmechanic
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Nicolai
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  • Related - http://physics.stackexchange.com/q/43072/ – Kitchi Dec 24 '13 at 13:17
  • http://hyperphysics.phy-astr.gsu.edu/hbase/astro/univacc.html maybe someone can explain the magnitude vs redshift plot in detail. If the universe is accelerating, supernova appear less bright than they ought to (less bright - more positive magnitude) – innisfree Dec 24 '13 at 13:49
  • Yes, the Hubble's law using red shift says that distance is greater that it is (detected by supernova bright). So red shift is greater than it must be. Why? Doppler effect - galaxies WAS running away a lot of billions years ago. But we do not know if it is so now. "Now" (2.5 million years ago) Andromeda is running to us. So more far galaxy - more far past. A lot of years ago galaxies was running away faster. The last that we can see is Andromeda. It is "running away" in a opposite direction. I see negative acceleration. Do You see negative acceleration? – Nicolai Dec 24 '13 at 14:29
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    I'm sorry, but was is the actual question? – Joe Hilton Dec 24 '13 at 13:08
  • My "banana" logic tell me that Universe isn't expanding but it is collapsing. Why scientists deduced that Universe is expanding? – Nicolai Dec 24 '13 at 13:13
  • Why do you think it is collapsing? – Joe Hilton Dec 24 '13 at 13:15
  • Far galaxies was running from us more faster. More nearly was running more slower. But Andromeda (most nearest) is running to us. From past to present speed was more and more slow. But 2.5+ million years ago speed changed direction. – Nicolai Dec 24 '13 at 13:18
  • Just because the Universe is expanding, doesn't mean that everything is moving away from one another. Many other galaxies might be on a collision course with each other – Joe Hilton Dec 24 '13 at 13:22
  • Yes, galaxies that are close each to other are falling to each other. Why You say that expanding is happening now? It was many years ago. We can see what was many years ago. If we look from far galaxies to closest we look from far past to nearest past. From other side we see that speed of galaxies are more and more slow (if we look from far galaxies to closest). So I deduced that speed is
    slowing down in time. Is not that logical?     – Nicolai Dec 24 '13 at 13:32
  • @Nicolai, you're failing to distinguish between gravitationally bound systems, such as the local group, and the large scale expansion of space on scales where the universe can be approximated as homogeneous and isotropic. See: https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric – Alfred Centauri Dec 24 '13 at 14:58
  • Let forget about Andromeda that is a part of our local group. Anyone says that Universe is expanding now, but this proposition is based on events that happened in a far past (they looked Ia supernovas from far galaxies). Without Andromeda we can see that speed of running away galaxies decreases with decreasing distance. But because of limited speed of light we can say that speed decreases with time. More far galaxies - more far past. More far galaxies - more fast speed => More far past - more fast speed. Is it not true? – Nicolai Dec 24 '13 at 15:29
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    Most of your questions are covered in my answer here: According to Hubble's Law, how can the expansion of the Universe be accelerating? – Pulsar Dec 24 '13 at 23:49

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It seems to me there are two separate questions here. The first is how we know distant galaxies are moving away from us when we can't see them now - we can only see them as there were some time ago. The second question is why we say the distant galaxies are accelerating away from us.

To answer the first question you need to know a bit how how physicists work. It isn't enough for us to measure the recession velocities of distant galaxies. We want an explanation for those velocities, that is a theory to describe how the galaxies move. We get this theory by starting with Einstein's equations of general relativity, making a few assumptions (homogeneity and isotropy) and solving the result equations to get an equation called the FLRW metric. This equation tells us how the galaxies should be moving, and when we look in our telescopes we find our observations match the predictions.

So we have a theory that gives the correct predictions for what a galaxy $x$ billion light years away was doing $x$ billion light years ago, and since the theory was correct then we assume it's still correct today. That's why we feel confident in saying what distant galaxies are doing right now, even though we can't see them doing it.

The second question follows on from the first, because the acceleration of the universe expansion is also explained by the FLRW metric, sort of.

The FLRW metric contains some adjustable parameters. The obvious one is the density of matter in the universe, but there is also an adjustable parameter called the cosmological constant and a non-zero cosmological constant will cause an acceleration in the expansion rate. It's important to be clear that (at the moment) the cosmological constant is just a parameter and we have no theory to say what its value should be. We have to compare observations with the predictions of the FLRW metric and choose the value of the cosmological constant that matches. When we look at how far away distant galaxies are we find that for the FLRW metric to match experiment we need a non-zero cosmological constant, and that's how we know that the expansion is accelerating.

John Rennie
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