The accelerating universe - Cosmic inflation

[BlackHole]

The standard big-bang cosmology is based on the cosmological principle, which assumes that the universe is homogenous and isotropic on large distances. We use the FRW metric.

 

Since cosmic inflation is getting slight support, why do cosmologists estimate that the acceleration began roughly 5 billion years ago? If so, what hapanned before that which caused the universe to decelerate?

[Martin]

The standard big-bang cosmology is based on the cosmological principle' date=' which assumes that the universe is homogenous and isotropic on large distances. We use the FRW metric.

 

Since cosmic inflation is getting slight support, why do cosmologists estimate that the acceleration began roughly 5 billion years ago? If so, what hapanned before that which caused the universe to decelerate?[/quote']

 

 

the assumption is that dark energy density is constant throughout all space and time at a value of about 0.6 joules per cubic kilometer.

 

it stays that density even while space expands! this is why people sometimes refer to it as the cosmological constant.

 

but matter thins out as space expands!

 

therefore 5 billion years ago, when the universe had not expanded so much, matter was much more concentrated and dark energy was relatively insignificant.

the attractive force of matter which slows expansion dominated!

 

up to 5 billion years ago (and people vary about when exactly, maybe changeover was only 1 or 2 billion years ago, but at least 5 billion years ago) was the MATTER DOMINATED period when matter was the dominate percentage of stuff and expansion was slowing.

 

but expansion continued enough so that matter got so thinned out that now dark energy is 73 percent and it dominates!

 

therefore because of the expansive effect of dark energy the expansion is accelerating.

 

Is that enough of an xplanation of the changeover or do you have more questions about it?

[J.C.MacSwell]

the assumption is that dark energy density is constant throughout all space and time at a value of about 0.6 joules per cubic kilometer.

 

it stays that density even while space expands! this is why people sometimes refer to it as the cosmological constant.

 

but matter thins out as space expands!

 

therefore 5 billion years ago' date=' when the universe had not expanded so much, matter was much more concentrated and dark energy was relatively insignificant.

the attractive force of matter which slows expansion dominated!

 

up to 5 billion years ago (and people vary about when exactly, maybe changeover was only 1 or 2 billion years ago, but at least 5 billion years ago) was the MATTER DOMINATED period when matter was the dominate percentage of stuff and expansion was slowing.

 

but expansion continued enough so that matter got so thinned out that now dark energy is 73 percent and it dominates!

 

therefore because of the expansive effect of dark energy the expansion is accelerating.

 

Is that enough of an xplanation of the changeover or do you have more questions about it?[/quote']

 

Isn't Dark Matter required as an attractive force within galaxies?

[J.C.MacSwell]

Was the evidence of the acceleration that the redhift was not "as expected" for the galaxies/clusters viewed at that distance/age?

[BlackHole]

the assumption is that dark energy density is constant throughout all space and time at a value of about 0.6 joules per cubic kilometer.

 

it stays that density even while space expands! this is why people sometimes refer to it as the cosmological constant.

 

but matter thins out as space expands!

 

therefore 5 billion years ago' date=' when the universe had not expanded so much, matter was much more concentrated and dark energy was relatively insignificant.

the attractive force of matter which slows expansion dominated!

 

up to 5 billion years ago (and people vary about when exactly, maybe changeover was only 1 or 2 billion years ago, but at least 5 billion years ago) was the MATTER DOMINATED period when matter was the dominate percentage of stuff and expansion was slowing.

 

but expansion continued enough so that matter got so thinned out that now dark energy is 73 percent and it dominates!

 

therefore because of the expansive effect of dark energy the expansion is accelerating.

 

Is that enough of an xplanation of the changeover or do you have more questions about it?[/quote']

 

So basically the universe was always accelarating until dark energy became dominant over dark matter and baryonic matter.

 

According to quintessence theory, dark energy has negative pressure (w < -1) which varies with space and time.

 

I guess something with our understanding of the universe is not correct but the most sophisticated thing is to found out what it is.

 

In Physicsweb there was a full article about dark energy with references.

[Martin]

Isn't Dark Matter required as an attractive[/b'] force within galaxies?

 

yes dark matter is attractive, and it can clump together like ordinary matter

and it can collect in galaxies and help to hold them together

 

dark energy is a completely different thing

 

the current best estimate based on WMAP data is that

dark energy density is 73 percent of total

dark matter density is 24 percent

baryonic ordinary matter and light etc is 4 percent

 

dark energy, by usual assumption, cannot clump

it is permanently spread out in a uniform constant density

(some people fudge around with types of dark energy which can clump a little bit but not very much, most working cosmologists just keep it simple and assume a constant uniform dark enrgy density, sort of like a "vacuum" energy that just belongs to space and is very very small like 0.6 joule per cubic kilometer---it is significant only because there is so much space)

[Martin]

Was the evidence of the acceleration that the redhift was not "as expected" for the galaxies/clusters viewed at that distance/age?

 

the original evidence (for dark energy or slightly positive cosmological constant)was data on SUPERNOVAS of a certain type called type Ia.

 

this data first appeared in 1998.

 

the supernovas were observed in distant galaxies at redshifts like

z = 0.7 actually a whole range of redshifts even some greater than 1.0.

 

It is a beautiful fact that type Ia SN (sometimes they abbreviate SNIa)

are all about the same brightness!

so one can tell their distance from their brightness

it is the old "standard candle" idea

and one can also accurately measure their redshift! (and I suppose if one wants also measure, as you suggest, the collective redshift of the galaxy in which they reside).

 

It is also wonderful, I think, that one can see individual stars in these distant galaxies. What a great feat of resolution. Of course it helps that they are supernovas so they are temporarily bright more or less like the galaxy itself.

[Martin]

Was the evidence of the acceleration that the redhift was not "as expected" for the galaxies/clusters viewed at that distance/age?

 

yes, exactly

it was basically just a curve fitting thing

(it was with SNIa data, not actually galaxies/clusters but that is a mere detail)

 

the plot of distance versus redshift did not fit the earlier model based on the Friedmann equation (derived from Gen Rel) with zero cosmological constant. so they had to introduce a slightly positive Lambda to get a fit.

[Martin]

So basically the universe was always accelarating until dark energy became dominant over dark matter and baryonic matter.

 

According to quintessence theory' date=' dark energy has negative pressure (w < -1) which varies with space and time.

 

I guess something with our understanding of the universe is not correct but the most sophisticated thing is to found out what it is.

 

In Physicsweb there was a full article about dark energy with references.

 

there are some small confusions here

 

So basically the universe was always accelarating until dark energy became dominant over dark matter and baryonic matter.

...

 

Actually expansion was mostly DEcelerating until dark energy became dominant.

(at the very beginning there was a supposed brief moment of inflation which is another kind of accelerating expansion, caused by something rather analogous to dark energy. negative pressure---dont know what it was---call it inflaton or scalar field)

But after that supposed brief instant of inflation it was all DEceleration for billions of years.

 

...

 

According to quintessence theory, dark energy has negative pressure (w < -1) which varies with space and time.

...

 

you dont need "quintessence" for that, I hear less and less about quintessence these days. I think there is a growing consensus that a simple constant dark energy or positive cosmological constant does the job as well or better.

 

dont confuse w<-1 with negative pressure

w = -1 exactly is also giving negative pressure (about the right amount)

 

I have seen recent observations that tend to exclude w < -1 cases and narrow things down closer and closer to w = -1

 

the good thing about it is a kind of Occam razor thing. you can get w =-1 very simply just with a constant vacuum energy, you dont need some new particle with weird unprecedented properties. a simple vacuum energy provides the negative pressure, accelerates expansion just the right amount, and fits the data.

 

...

In Physicsweb there was a full article about dark energy with references

 

Great! thanks for providing a reference. I hope it is a recent one. We probably have some more references for dark energy in the Astronomy Reference sticky thread that a good librarian made for us at SFN.

[Jacques]

the assumption is that dark energy density is constant throughout all space and time at a value of about 0.6 joules per cubic kilometer.

 

it stays that density even while space expands! this is why people sometimes refer to it as the cosmological constant.

If it stays constant while space expands, doesn't it mean that dark energy is constanly created from nothing ?

[Cadmus]

(at the very beginning there was a supposed brief moment of inflation which is another kind of accelerating expansion' date=' caused by something rather analogous to dark energy. negative pressure---dont know what it was---call it inflaton or scalar field)

But after that supposed brief instant of inflation it was all DEceleration for billions of years.[/quote']This notion of a brief instant is popular. However, it assumes that time has always and everywhere been as we understand it here and now. I think that this is not the case, and that this brief instant encapsulates all of the time that is outside of the context that we recognize it in modern science in this part of the unvierse.

[Johnny5]

The standard big-bang cosmology is based on the cosmological principle' date=' which assumes that the universe is homogenous and isotropic on large distances. We use the FRW metric.

 

Since cosmic inflation is getting slight support, why do cosmologists estimate that the acceleration began roughly 5 billion years ago? If so, what hapanned before that which caused the universe to decelerate?[/quote']

 

I read this recently in a paper at the IOP. How did they conclude that the universe is only 5 billion years old?

[Martin]

I read this recently in a paper at the IOP. How did they conclude that the universe is only 5 billion years old?

 

they didnt, the consensus about universe age is 13.7 or about 14 billion years.

 

expansion started 14 billion Y ago but it started off very fast and was decelerating for the first 9 or 10 billion years

 

then, about 4 or 5 billion Y ago this deceleration stopped and the expansion began to pick up again

 

this means that when you plot the scale factor (loosely called "the size of the universe") the curve has a convex portion and concave portion and an inflection point

 

there should be a picture of that in the SFN Astronomy Reference sticky thread

 

the plot of the scale factor is basically just an increasing ramp showing it getting larger but superimposed on that is a mild S-curve where for a while the slope is decreasing and then it starts increasing

 

Charlie Lineweaver, a top cosmologist (who just had a piece in SciAm BTW) has a picture of that in his survey article "Inflation and the CMB". good article written clearly for general audience but uses math and doesnt oversimplify

 

yeah here's from that stickythread

 

This article by Lineweaver (he was one of the team in charge of COBE

an earlier CMB satellite observatory)

 

"Inflation and the Cosmic Microwave Background"

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver_contents.html

 

http://arxiv.org/astro-ph/0305179

 

the second is more legible.

 

LOOK AT FIGURE 14 "the size and destiny of the universe"

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_7.html

 

this is the picture I wanted. it shows expansion is always going on for 14 billion years but for a while it slows and then it begins to speed up

 

here is figure 14 in more detail

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure14.jpg

[J.C.MacSwell]

they didnt' date=' the consensus about universe age is 13.7 or about 14 billion years.

 

expansion started 14 billion Y ago but it started off very fast and was decelerating for the first 9 or 10 billion years

 

then, about 4 or 5 billion Y ago this deceleration stopped and the expansion began to pick up again

 

this means that when you plot the scale factor (loosely called "the size of the universe") the curve has a convex portion and concave portion and an inflection point

 

there should be a picture of that in the SFN Astronomy Reference sticky thread

 

the plot of the scale factor is basically just an increasing ramp showing it getting larger but superimposed on that is a mild S-curve where for a while the slope is decreasing and then it starts increasing

 

Charlie Lineweaver, a top cosmologist (who just had a piece in SciAm BTW) has a picture of that in his survey article "Inflation and the CMB". good article written clearly for general audience but uses math and doesnt oversimplify

 

yeah here's from that stickythread

 

This article by Lineweaver (he was one of the team in charge of COBE

an earlier CMB satellite observatory)

 

"Inflation and the Cosmic Microwave Background"

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver_contents.html

 

http://arxiv.org/astro-ph/0305179

 

the second is more legible.

 

LOOK AT FIGURE 14 "the size and destiny of the universe"

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_7.html

 

this is the picture I wanted. it shows expansion is always going on for 14 billion years but for a while it slows and then it begins to speed up

 

here is figure 14 in more detail

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure14.jpg[/quote']

 

Is the Big Bang a Theory or just a Model? I know there is some degree of circumstantial evidence but other than the Cosmic Background Radiation none of it was predicted and the "model" has to changing to fit new data.

[Martin]

Is the Big Bang a Theory or just a Model? I know there is some degree of circumstantial evidence but other than the Cosmic Background Radiation none of it was predicted and the "model" has to changing to fit new data.

 

the theory here is Gen Rel, hasnt changed since 1915 and keeps passing all the tests people can think up to the highest accuracy they can measure

 

 

Friedmann derived the Friedmann equation around 1922 or so (forget when) straight out of Gen Rel by making a simplifying assumption that on large scale the matter is distributed evenly, this is an approximation, not a theory.

 

it seems to be roughly homogeneous and isotropic

and assuming that makes the calculation ever so much easier

 

AFAIK the "big bang" picture is just the Friedmann equation. It has not changed for some 80 years.

 

they have refined the PARAMETERS that they plug into it

which has basically a business of narrowing down

 

Lambda, in 1998, was found not to be exactly zero but to be a very small positive quantity

 

they made a big deal of it but it didnt change the theory, it just refined it by narrowing down on better and better values of the half-dozen numbers they need to plug into the Friedmann equation to make it go.

 

Friedmann is basically a convenient simplification of Gen Rel, which is the underlying theory of how spacetime is shaped by matter----it is our prevailing theory of, for example, gravity. I dont know of any serious competition.

[BlackHole]

I don't know but leaking gravity could solve the 'dark energy' puzzle. That's what i read on space.com. According to (super)string theory, the universe has more dimensions than the four we experience in every-day life and gravity may leak away to one of the other 6 microscopic dimensions.

 

Another proposal is that dark energy and neutrinos might be connected.

[J.C.MacSwell]

. I dont know of any serious competition.

 

Does this make it a "best guess or a Theory (capital "T").

 

Also, what would you bet that it is correct (odds not $)

 

I'm at 50% and dropping. I am at heart a Steady State guy (brain only allows 25% but it's rising) but have no consistent mathematical model to base it on. It is not even at the "half baked" stage and could end up closer to Big Bang (which I'm betting is twice as likely at this point).

 

I appreciate your posts. They are informative, challenging and fun. I wish I had more time to learn the math at the level of some of the posters here.

[BlackHole]

Is there any chance that the accelerating expansion of the universe is an illusion caused by relativistic redshift in stationary clouds of dust (see paper)?

 

Such theories are unlikely because observations have almost firmly confirmed that the universe is accelerating based on CMB. The ΛCDM model is the best candidate.

 

However cosmic inflation theory is still suspect. The graceful-exit problem is still not solved and again the only solution is string theory.

[Johnny5]

Friedmann is basically a convenient simplification of Gen Rel' date=' which is the underlying theory of how spacetime is shaped by matter----it is our prevailing theory of, for example, gravity. I dont know of any serious competition.[/quote']

 

Martin, is general relativity a field theory?

[J.C.MacSwell]

Is there any chance that the accelerating expansion of the universe is an illusion caused by relativistic redshift in stationary clouds of dust (see paper)?

 

Such theories are unlikely because observations have almost firmly confirmed that the universe is accelerating based on CMB. The ΛCDM model is the best candidate.

 

However cosmic inflation theory is still suspect. The graceful-exit problem is still not solved and again the only solution is string theory.

 

String theory is the only one that explains what happened on the grassy knoll also. One particular solution (among many others) has Fidel passing Mother Teresa a loaded gun.

[BlackHole]

String theory is the only one that explains what happened on the grassy knoll also. One particular solution (among many others) has Fidel passing Mother Teresa a loaded gun.

 

I think i get the meaning (i'm not sure). String theory is strong but still baseless. The fact that it is almost solely based upon the flawed ancient notion of the discrete particle. The work of Einstein, Schrodinger, Clifford, Dirac proposes that matter must be a wave structure, continuous in the space it occupies (or is part of). To their view, our particle concepts are a result (not the cause) of the particle-like 'appearances' of the wave structures.