Unfortunately, I originally posed this question in the wrong place where it has gone undetected and unanswered. Hopefully it will now grab somebody's attention and quench my curiosity.

The quetion I have regards the accelerating expansion of the universe. I may be misinformed, but from what I understand the theory is based on observations showing that the more distant a galaxy is from us, the faster it is moving away from us. If that is correct, then wouldn't that show the opposite to be true? Wouldn't that demonstrate that the expansion of the universe is slowing down?

When observing a distant objects you are looking back in time. For instance, let's say an object 10 million light years away is moving away from us at (I don't know the actual numbers so consider them as hypothetical) 35 km/sec and an object 100 million light years away is at 50 km/sec. Wouldn't you actually be observing that the rate of expansion was 35 km/sec 10 million years ago and 50 km/sec 100 million years ago?

Essentially I am asking how we can observe a billion year old circumstance and extrapolate what is happening right now?

This is evidence that the universe is simply expanding. Acceleration of the expansion is based on other observations.

No. Distant objects are moving away from us (in all directions) because space itself is expanding. So something twice as far away appears to be receding twice as fast because, well, there is twice as much expanding space between here and there.

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Everyone is entitled to his own opinion, but not his own facts.

The reason we think the metric expansion of space may be accelerating is due to our interpretation of observations of type 1a supernovae from the 1990's, an interpretation which has since been corroborated by measurements of the Cosmic Microwave Background Radiation from the WMAP project and by more accurate recent measurements of those supernovae.

We use cosmological redshift to measure how the space in between us and a certain object has expanded since the light we are measuring was emitted, but in order to ascertain an accurate measure of that object's distance we need something else - something to calibrate our measurements to: An object for which we know the actual brightness, or absolute magnitude for. Once we have this object, known by astronomers as a standard candle or cosmic candle, we can then compare the object's apparent magnitude to it's absolute magnitude. Only with standard candles can we use Hubble's Law to show the relationship between redshift and distance.

Currently, the best standard candles known to astronomers are type 1a supernovae. When we compare their apparent magnitude to their redshift we can build up a picture of how the universe expanded, and this picture shows us that the expansion of space seems to be unexpectedly accelerating. Basically, the supernovae seem dimmer than they should be, but the amount that they are dimmer than they should be changes depending on their distance (or their time in the past).

Unfortunately, neither of you actually addressed what my confusion is based on. Observations that are used to explain the expansion are referred to as being in the present. However, nothing we observe at cosmological distances are happening right now.

"Distant objects are moving away from us "

They were moving away from us when the photons they emitted finally reached us, but I know of no way to observe their current behavior.

I am aware of the type 1a supernova observations, yet they still are not "live" observations. They are historical observations.

Maybe I should rephrase my original question. If we cannot observe what is actually happening in a galaxy far, far away right now, how can we extrapolate that the universe is expanding at an accelerated rate right now?

Actually, while a little hard to picture, the redshift represents current velocity, not the velocity when the light was emitted. Say an object is 100 million light years away and receding at 50 km/sec, as you suggest. The object was closer--say 80 million light-years away when the light was emitted 100 million years ago--and it was receding slower, say 40 km/s. In the intervening years, the distance of 80 mly expanded to 100 mly, and its recession velocity increased from 40 to 50 km/s.

Today, when we observe it, the object's present velocity and present distance are reflected in the observed redshift and magnitude of the object, respectively.


Does that answer your question?

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PW -- Plant Whisperer

Yes, I see how I might have missed your point, sorry about that!

But as you were originally asking if the observation that the further away things are, the faster they are moving away from us (Hubble's law) actually meant the expansion was slowing down, I assumed you weren't aware that it was the observations of the type 1a supernova that was implying the acceleration.

Imagine (for example - I will use arbitrary figures) that an object we have an accurate distance for is 1 billion ly away and has a cosmological redshift of 1. Then imagine an object that is 2 billion ly away has a cosmological redshift of 1.9. This would mean the light was stretched by the expansion of space by 0.9 between 2 billion and 1 billion years ago, and was stretched a further 1.0 between 1 billion years ago and now, and thus the expansion was accelerating when the light left the closer object compared to the more distant one.

Remember that unlike relativistic redshift, which is a redshift perceived due to relativity and doppler effect between 2 objects in inertial motion and the light itself isn't actually changed in any way, cosmological redshift is an actual "stretching" of the light due to it's travelling though space which is expanding whilst it travels through it. The effect of cosmological redshift is cumulative.

It is only cosmological redshift that tells us about the expansion of space, where the spectrum is shifted due to being actually changed, rather than the apparent changes to the light caused by inertial movement.

We cannot measure cosmological redshift (and thus measure the expansion of space) at close distances, as the amount that expanding space would shift the spectrum over such a relatively short distance in cosmic terms is tiny in comparison to the redshift imparted by relativistic doppler effect. But over larger distances the cosmological redshift becomes the dominant component of an objects redshift.

So if a standard candle 10 billion ly away has a redshift of 7.9 and a standard candle 5 billion ly away has a redshift of 4, over those distances the amount of redshift from inertial movement of those objects made is absolutely tiny in comparison to the amount of redshift from the expansion of space. After a number of observations of the type 1a supernovae over differing distances we built up this picture of accelerating expansion.

If it seems to be accelerating up to the closest point we can be confident of measuring for it, and we have no evidence to assume it started decelerating or remained constant, we can only assume that more recently the expansion is still accelerating.

Perhaps studying this site will help your perspective.

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Everyone is entitled to his own opinion, but not his own facts.

Cleared up. Thanks all.