Universe expansion really speeding up?

[deklore]

All scientists now confirm that the speed of expansion in the universe is increasing.

However, if scientists are using redshift to calculate this then the light they are using may have traveled millions of light years.

So my query is, if that is the case, then couldn't the universe be slowing down, but the evidence (the light) that proves this has yet to reach our eyes?

 

[Smock]

I completely agree. We can only work with the information we have and maybe in the next hundred years we will see that the universe is oscillating and not actually increasing speed of expansion infinitely. There is no way of knowing and all we can do is wait to find out. Based on the data now, it does seem as though our universe is increasing speed of expansion.

 

[bcrowell]

We can only work with the information we have and maybe in the next hundred years we will see that the universe is oscillating and not actually increasing speed of expansion infinitely.

A hundred years is nothing compared to cosmological timescales. The type of "information" you're referring to is different from the type of information the OP was referring to.

There are certain types of limits on our knowledge that are fundamentally imposed by the laws of physics. Relativity limits us to knowing about our past light-cones. The Heisenberg uncertainty principle limits us as well. These are *fundamental* limits, which is what #1 was about.

 

[Chalnoth]

All scientists now confirm that the speed of expansion in the universe is increasing.

However, if scientists are using redshift to calculate this then the light they are using may have traveled millions of light years.

So my query is, if that is the case, then couldn't the universe be slowing down, but the evidence (the light) that proves this has yet to reach our eyes?

The way to answer this is to look at very distant supernovae. In the standard model of cosmology, with a cosmological constant explaining the expansion, the accelerated expansion is a recent phenomenon. If we look far enough back, we should see deceleration.

And that's exactly what we see: while medium-distance supernovae are dimmer than we would expect without acceleration, very far away supernovae start to get brighter than we would expect with no past deceleration.

This demonstrates that whatever is causing the apparent acceleration cannot simply be due to something happening to the light between us and the supernovae (e.g. dust that obscures the light between us and the supernovae that appear dimmer).

 

[Tanelorn]

All scientists now confirm that the speed of expansion in the universe is increasing.

However, if scientists are using redshift to calculate this then the light they are using may have traveled millions of light years.

So my query is, if that is the case, then couldn't the universe be slowing down, but the evidence (the light) that proves this has yet to reach our eyes?

Deklore, you make an extremely good point. Red shift is the most accurate way of estimating the speed of a supernova receding away from us and so it would provide very strong evidence on whether the expansion of the universe is accelerating. However we would have to measure another supernova in the same galaxy a significant amount of time (millions of years) later to be able to verify this.

If we instead rely on the attenuation of light intensity I would suggest that it is possible for the attenuation of light to be lower in the early days of the universe and to increase until the present day. I am not sure if this might be used to explain the variation that Chalnoth mentions.

 

[Chalnoth]

Deklore, you make an extremely good point. Red shift is the most accurate way of estimating the speed of a supernova receding away from us and so it would provide very strong evidence on whether the expansion of the universe is accelerating. However we would have to measure another supernova in the same galaxy a significant amount of time (millions of years) later to be able to verify this.

Or you can just measure the spectrum extremely accurately. And the nice thing is, we don't need a supernova to measure a spectrum. The galaxy itself is good enough to get the redshift. From what I understand, there are proposals in place to measure extremely precisely the change in redshift with respect to time.

 

[Tanelorn]

Or you can just measure the spectrum extremely accurately. And the nice thing is, we don't need a supernova to measure a spectrum. The galaxy itself is good enough to get the redshift. From what I understand, there are proposals in place to measure extremely precisely the change in redshift with respect to time.

That is very interesting and I think it makes a lot of sense to do this. A starting point would be an extremely accurate repeatable clock oscillator (perhaps a quasar? or an atomic clock?) of some kind to capture the signal or perhaps to use the red shifted source itself as a reference to find the frequency delta some time later. I would think that there would be a great many people looking for a successful measurement of this. And the longer we wait to measure it again the greater the accuracy and certainty.

Perhaps the light from a distant supernova might travel along differnet paths perhaps due to lensing and one path might be delayed by a year or so? Perhaps the 1 extra year of travel is enough to measure a change in red shift? I guess the main problem with this is that there is only one supenova per galaxy every 200 years and not all are the right type!

 

[Chalnoth]

That is very interesting and I think it makes a lot of sense to do this. A starting point would be an extremely accurate repeatable clock oscillator (perhaps a quasar? or an atomic clock?) of some kind to capture the signal or perhaps to use the red shifted source itself as a reference to find the frequency delta some time later. I would think that there would be a great many people looking for a successful measurement of this. And the longer we wait to measure it again the greater the accuracy and certainty.

Perhaps the light from a distant supernova might travel along differnet paths perhaps due to lensing and one path might be delayed by a year or so? Perhaps the 1 extra year of travel is enough to measure a change in red shift? I guess the main problem with this is that there is only one supenova per galaxy every 200 years and not all are the right type!

Well, supernovae are really poor for this sort of measurement, because as you mentioned, they're rare. But since all objects in the galaxy should follow the expansion, all we need is an object with bright emission lines of some sort or other. And for that, we can indeed find a number of strongly-lensed sources (sources for which we see many images). I doubt we'll ever find a light travel time difference as great as a year, though. From what I understand, usually these differences are on the order of seconds. It's somewhat difficult to measure precisely, though, because in general we don't know the mass distribution of the gravitational lens, and so cannot calculate the time difference. That has to be measured through observation.

 

[deklore]

Apologies for sounding thick here, but i need to put it in laymans terms.

Going by what I've read so far, is it true to say that the expansion of the far reaches of the universe could be slowing down, even changing to collapse, but we have no way of knowing it yet?

If that is the case then why are scientists declaring the universe expansion is increasing when it may not be the case. Kinda like seeing its sunny in your part of the world and then declaring there is no rain in the entire world. How can they make such generalised statements when they don't have all the facts, just what they have observed so far?

 

[Chalnoth]

Apologies for sounding thick here, but i need to put it in laymans terms.

Going by what I've read so far, is it true to say that the expansion of the far reaches of the universe could be slowing down, even changing to collapse, but we have no way of knowing it yet?

If that is the case then why are scientists declaring the universe expansion is increasing when it may not be the case. Kinda like seeing its sunny in your part of the world and then declaring there is no rain in the entire world. How can they make such generalised statements when they don't have all the facts, just what they have observed so far?

a) In general, when talking about the behavior of "the universe" scientists usually only mean the observable universe.
b) Expansion is driven by the density of the various forms of matter/energy that make up the universe. So far as we can tell, the density of each sort of matter/energy in the observable universe is the same everywhere.

 

[Skolon]

What did we know about acceleration of Universe (a'(t)) and its future?
The second time-derivative a''(t) is positive, negative or zero?

From what I read until now I can suppose a''(t) is positive so a'(t) will continue to increase in time forever. Right?

 

[Chalnoth]

What did we know about acceleration of Universe (a'(t)) and its future?

Well, it's difficult to predict the future with confidence. The difficulty is that we cannot say for certain what the causes of this acceleration are. Without knowing the causes, we can't be sure about the future history.

However, if the simplest model is correct, that the cause is a cosmological constant or something that behaves very much like one, then yes, $\ddot{a}$ and $\dot{a}$ will both remain positive forever.

What happens is that as the universe expands, the matter and radiation continue to dilute, but the cosmological constant remains at the same density no matter the expansion. Then we look at the first Friedmann equation:

$$H^2 = {8 \pi G \over 3} \rho$$

and we see that if the normal matter and radiation decay away, and all we are left with is an energy density that is constant and doesn't change in time, then we approach a situation where the Hubble parameter $H(a)$ approaches a constant value. Now, the Hubble parameter is defined as:

$$H(a) = {\dot{a} \over a}$$

So what happens is that this ratio of the time derivative of the scale factor to the scale factor itself gets smaller and smaller as the radiation and matter dilute away, approaching a constant value, let's call it $H_f$ (f for future). So in the distant future, we have:

$$H(a) = H_f$$

If we evaluate this in terms of the definition of the Hubble parameter, we have a very simple differential equation:
$$\dot{a} = H_f a$$

Which becomes:
$$a(t) = a(0)e^{H_f t}$$

So if the dark energy acts as a cosmological constant into the future, then the future is exponential expansion that never ends.

The difficulty is that we can't be certain what the cause of the accelerated expansion is just yet, so we can't say for sure one way or the other whether this is the future of our universe or not. By virtue of being the simplest explanation, it is perhaps the most likely, but that isn't very convincing to many, given how little we know about dark energy's nature.

 

[Skolon]

The difficulty is that we can't be certain what the cause of the accelerated expansion is just yet, so we can't say for sure one way or the other whether this is the future of our universe or not. By virtue of being the simplest explanation, it is perhaps the most likely, but that isn't very convincing to many, given how little we know about dark energy's nature.

Thanks, Chalnoth. So the answer are in a deep understanding of dark energy.

One more question please: what did we certainly know about dark energy, other then its value?
This energy is caused by internal conditions (from observable Universe) or can be an effect of something from "outside" Universe?
And how we can improve this dark energy knowledge? What are the ways for a future better understanding?

 

[Chalnoth]

One more question please: what did we certainly know about dark energy, other then its value?

Well, we know it has to explain the observed expansion. That requires that it varies very little, if at all, in energy density as the universe expands. In the coming years, we will be obtaining more data about how structure (galaxies, galaxy clusters, etc.) formed in the early universe. This formation of structure depends in a somewhat different way on the behavior of dark energy, and that will give us (hopefully) a better picture of its nature.

This energy is caused by internal conditions (from observable Universe) or can be an effect of something from "outside" Universe?

Well, it basically can't be from anything "outside". Speed of light limitations prevent that.

 

[Skolon]

Well, it basically can't be from anything "outside". Speed of light limitations prevent that.

Yes, but only if all is "outside" exist just after Big Bang. If somehow an outside influence was present before BB than maybe this can be a good explanation for dark energy.
But, this idea is against all BB are saying: "outside space" and "before BB" not exist. I don't know if this concepts can be integrated on The Big Bang Theory and remain a viable theory, I suppose not.

 

[Ut-Napishtim]

"couldnt the universe be slowing down, but the evidence (the light) that proves this has yet to reach our eyes? "

Deklore:

You have made an excellent point - We need to wait just only a couple of billion years to know FOR SURE what is happenning NOW at the far distant places of the Universe, and there is NO way around it (unless, in a very unlikely event, the whole space of the Universe collapses drastically tomorrow around the Earth and brings with it the weary from a long travel photons).

I am a new here and would very much appreciate to know - Where are the instructions for using these forums, e.g. - How to insert quotations from messages, what is the meaning/use of various icons etc.

Thanks for attention

 

[PacketMan]

Yes, but only if all is "outside" exist just after Big Bang. If somehow an outside influence was present before BB than maybe this can be a good explanation for dark energy.
But, this idea is against all BB are saying: "outside space" and "before BB" not exist. I don't know if this concepts can be integrated on The Big Bang Theory and remain a viable theory, I suppose not.

M-theory posits that rippling membranes floating in 11 dimensional space existed before the big bang, and that big bangs possibly arise from them bumping into each other. So M-theory is a marriage of Big Bang theory and "outside space before BB".