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It is said, that by measuring distant sources, we can gather information about the past state of the universe. The further we measure, the faster things appear to recede, which brings me to my question, which has puzzled me literally for years.

If distant sources tell something about the past, would that not mean the universe is slowing down today? If not, how come?

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    $\begingroup$ Aren't you forgetting that the most basic characteristic of an expanding universe is that distant galaxies recede faster than closer ones? Whether the expansion is accelerating or decelerating is a minor point compared to that & requires careful measurements & analysis to determine. $\endgroup$ – D. Halsey Apr 1 at 12:51
  • $\begingroup$ I didn't forget this, I already said in OP that distant galaxies recede faster. That coupled with the truth that most distant galaxies are events that happened in the past. So maybe re-read the OP again? $\endgroup$ – Gareth Meredith Apr 1 at 15:02
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    $\begingroup$ @GarethMeredith Why you think that observing the past would mean the universe is slowing down ? $\endgroup$ – Reign Apr 4 at 8:36
  • $\begingroup$ Well, simply because the further we look into space the further we look back in time. The more we go back in time, the faster the universe has to expand. $\endgroup$ – Gareth Meredith Apr 4 at 8:53
  • $\begingroup$ unclear what I am asking? Other people are following this fine? $\endgroup$ – Gareth Meredith Apr 8 at 10:47
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If distant sources tell something about the past, would that not mean the universe is slowing down today? If not, how come?

If in the past they are accelerating they should accelerate now too. Since we cannot expect that galaxies will slow down and come towards us at some point.

I want to talk about two things.

I-Redshift Observation

The EM spectrum of the galaxies shows us that, galaxies are receding from us And in cosmology, redshift observation is independent of time. For instance, we are observing a galaxy that is 100Mpc away. Which means we are observing 30 million years ago in time and we see that the galaxy is redshifted. Since in time it's not possible to expect that the galaxy will slow down and coming back to us, I think we can safely assume that it will be redshifted forever. Because from the observational data it seems that the further they are their velocity is higher. By observing the spectrum we can determine the speed of the galaxies or light sources. And it seems that they are accelerating.

II- The Deceleration Parameter

We can measure the density of the matter, and dark energy by modeling our universe. By using Friedmann Equations and by calculating density parameters we can understand the evolution of the universe. Now I want to talk about the deceleration parameter.

The deceleration parameter can be written as,

$$q_0=1/2\Omega_{m,0}-\Omega_{\Lambda}+\Omega_{r,0}$$

If $q_0>0$ it means that the universe is decelerating and if $q_0<0$ the universe is accelerating. By setting the current values of $\Omega_{m,0}=0.30$, $\Omega_{\Lambda}=0.70$ and $\Omega_{r,0}\approx0$

$$q_0\approx-0.55$$

And our observations and data best fit shows us that $q$ is negative and close to $-0.55$

So by looking at the CMBR data and observing the supernovas, we can see that the universe is accelerating.

The universe is accelerating because the dark energy density will become dominant over time. Since the matter density drops like $a^{-3}$ however the dark energy is independent of $a$ (where $a$ is scale factor). And this is currently what is happening.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – ACuriousMind Apr 8 at 19:57
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The reason that distant sources tell us something about the past is because light takes time to travel to us. It might be easiest to explain using a real-life example:

Suppose you're watching a distant thunderstorm. Occasionally, you see lightning flash and, a few seconds later, you hear a thunderclap. The big difference between the timing of the flash and the thunderclap is precisely because sound has, even on human scales, a finite propagation speed, while over these distances, light travels from the flash to our eyes essentially instantaneously (within a few microseconds). In this case, you can say that we are hearing the sound from a lightning strike that happened a few seconds in the past, which we can easily confirm by comparing the flash (which essentially reaches our eyes right as it happens) to the thunderclap (which reaches our ears a few seconds later). Clearly, this isn't because time is running slower for the thunderstorm than it is for us; rather, it's because we're far enough away that the sound takes a significant amount of time to travel to us.

What may be confusing you, in the cosmological case, is that we are, in this analogy, blindfolded. When we study distant galaxies, the distances are large enough that even light takes a significant amount of time to reach us, and so light takes the place of the thunderclap, and nothing takes the place of the lightning flash, because nothing can travel faster than light. So we're entirely relying on our cosmological "ears" to examine what happens in galactic "thunderstorms" at these vast distances, meaning that all of the information we gather comes from the distant past (since light that is reaching us now must have left those galaxies in the distant past in order for it to have enough time to get here).

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  • $\begingroup$ That's right, light takes time to reach us, which brings about the same problem, because that signal would still be telling us about the state of the universe in the past - at best, you can say it was accelerating in the past, but there is no evidence of acceleration from a local measurement. $\endgroup$ – Gareth Meredith Apr 1 at 18:34
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    $\begingroup$ @GarethMeredith How does your question, as currently asked, have anything to do with the accelerating expansion of the universe? $\endgroup$ – probably_someone Apr 1 at 18:55
  • $\begingroup$ Are you being serious? $\endgroup$ – Gareth Meredith Apr 10 at 15:01

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