# How do we know that the universe is still expanding

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This idea has always bugged me:
If we are looking at the past when observing the redshift of far away galaxies (ex: 10 billions light years).. then how can we tell that the universe at the present time is still expanding at same rate, deaccelerated, or stopped expanding?

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PeroK
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This idea has always bugged me:
If we are looking at the past when observing the redshift of far away galaxies (ex: 10 billions light years).. then how can we tell that the universe at the present time is still expanding at same rate, deaccelerated, or stopped expanding?

Technically, we can't. But, if the past data shows a pattern, then we can extrapolate that data.

However, there is also data on galaxies much closer than 10 billion light years away. It's putting all that data together that gives the expansion over time.

E.g.

"For supernovae at redshift less than around 0.1, or light travel time less than 10 percent of the age of the universe, this gives a nearly linear distance–redshift relation due to Hubble's law. At larger distances, since the expansion rate of the universe has changed over time, the distance-redshift relation deviates from linearity, and this deviation depends on how the expansion rate has changed over time. The full calculation requires computer integration of the Friedmann equation ..."

From:
https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_universe#Evidence_for_acceleration

Orodruin
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It is not only about the data showing a trend. It is about that trend fitting well with our current best models of how the Universe works and what that very same model, given the data, tells us will happen in the future.

PeroK
It is not only about the data showing a trend. It is about that trend fitting well with our current best models of how the Universe works and what that very same model, given the data, tells us will happen in the future.
Please explain to me how does it fit.. as far as I know the "accelerating" expansion does not fit with GR.

Delta2
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Please explain to me how does it fit.. as far as I know the "accelerating" expansion does not fit with GR.

The theory of an expanding universe originated with Einstein's GR. An accelerated expansion is consistent with there being a non-zero vacuum energy. In any case, the cosmological models are based on GR.

Technically, we can't. But, if the past data shows a pattern, then we can extrapolate that data.

However, there is also data on galaxies much closer than 10 billion light years away. It's putting all that data together that gives the expansion over time.

What if closer galaxies tell us that older galaxies have slowed down? In other words the expansion rate we see at 5 billion years way, is the future expansion rate for the 10 billion years away galaxies?

PeroK
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What if closer galaxies tell us that older galaxies have slowed down? In other words the expansion rate we see at 5 billion years way, is the future expansion rate for the 10 billion years away galaxies?

That model wouldn't fit either the data or the theory. In particular, I think you are misinterpreting the meaning of "expansion rate". Although galaxies further way (and further in the past) are receding faster than nearer galaxies, the expansion rate (recessional velocity per unit distance is increasing with time, and therefore with proximity).

If expansion were slowing down, we would see a very different pattern, if that is your question. Essentially the opposite of what we do see.

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Orodruin
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