Dark Energy, Dark Matter - One and the Same?

[Inevitable Droid]

This was a good article that pulled a lot of things together for me:  Dark Energy, Dark Matter - One and the Same? - http://www.space.com/scienceastronomy/mystery_monday_040712.html

This is another article worth reading, I think: Dark Energy, Dark Matter - http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/

[Cycel]

This was a good article that pulled a lot of things together for me:  Dark Energy, Dark Matter - One and the Same?

Interesting, but I am more confused than ever.

What if space is simply expanding, carrying everything apart.  No dark energy would be needed to account for the observations, or am I wrong?  I rather like Fred Hoyle's notion of a Steady State universe.  I wonder if there is anyway to incorporate Steady State ideas into current observations?  Something about the Big Bang just doesn't sit right with me, but that's just a gut feeling with little to back it up.  I think I heard somewhere that a few cosmologists are taking a second look a Hoyle's ideas.  Do you know anything about this?

[Inevitable Droid]

What if space is simply expanding, carrying everything apart.  No dark energy would be needed to account for the observations, or am I wrong?

I'm sure hackenslash can provide the definitive answer.  As for me, being an outsider looking in, my guess, strictly a guess, is that we're talking about acceleration, and acceleration generally requires energy, so it made sense to physicists to talk about energy.  Similarly, in the case of dark matter, we're talking about gravity, and gravity is mass pulling mass, so it made sense to physicists to talk about mass, hence matter.  Just as I personally would call dark matter WTF*, so would I personally call dark energy OMG**.  

* Weird Tug Factor

** Odd Movement Generator

I rather like Fred Hoyle's notion of a Steady State universe.  I wonder if there is anyway to incorporate Steady State ideas into current observations?  Something about the Big Bang just doesn't sit right with me, but that's just a gut feeling with little to back it up.  I think I heard somewhere that a few cosmologists are taking a second look a Hoyle's ideas.  Do you know anything about this?

From what I understand, Steady State Theory fails on three fronts:

1. Its main impetus is the notion that conditions in the universe in the super-distant past were basically the same as now - and that turns out to be false, as there are astronomical entities that emerged in the super-distant past that don't seem to be popping up nowadays, examples being quasars and radio galaxies.

2. Steady State Theory has no explanation for the abundance of deuterium and helium-3, whereas the Big Bang Theory does.

3. Steady State Theory doesn't predict the microwave background radiation, whereas the Big Bang Theory does.

[Cycel]

From what I understand, Steady State Theory fails on three fronts....

Ah well, back to the Big Bang I guess.

Did you see this story: Cosmos may show echoes of events before Big Bang?  I appeared today on the BBC news.  According to the article evidence has been found that the Big Bang may be cyclical.

 http://www.bbc.co.uk/news/science-environment-11837869

[Cycel]

Just as I personally would call dark matter WTF*, so would I personally call dark energy OMG**.

[grim-reaper]

Einstein already showed that ordinary matter is a form of energy. So it's possible that dark matter is a form of dark energy, or that both are different aspects of some still-unrevealed entity. One problem with all of this is that dark matter is apparently trying to pull the universe together whereas dark energy is apparently pushing it apart. So there still isn't a sure answer.

[Asmodean]

Could it be that stuff actually IS "just flying apart"..? The amount of kinetic energy achieved by matter in the Bang had to be... Well, of cosmic proportions... So are we sure we are not in that phase where the bullet has not yet left the barrel, so to speak..? Where the momentum of our charge has not yet been fully countered by the ever-weakening gravitational bonds between the galaxies flying apart..?

Just speculating - that particular area of space sciences is not exactly something I've studied much.

[hackenslash]

What if space is simply expanding, carrying everything apart.  No dark energy would be needed to account for the observations, or am I wrong?

Weeelllll...

This is a thorny issue, as I've said before. The problem with this idea is really that expansion would have to be a force in its own right, which leaves us where we are now. The reason that dark energy is postulated is because the expansion of the cosmos is accelerating, which is why expansion is then required to be a force. It simply can't be expansion due to energy input at the big bang, because that could only account for the initial expansion, after which expansion would continue at a reasonably steady rate. This means that there has to be something driving expansion. We call this dark energy, or the cosmological constant.

What has been postulated is a super-cooled Higgs field, which accounts for all expansion, including the initial inflation phase. The best and most accessible analogy to describe this that I've come across is the idea of a bowl with a slightly raised plateau in the middle. As long as the Higgs field is non-zero, it can have an effect on the expansion of space. During the initial inflation, you can think of the value of the Higgs field as sitting up on the edge of the bowl, but as expansion continued, it slipped down into the middle of the bowl. In a normal bowl, it would reach a zero-value when it got to the bottom. In our bowl, though, we have a slightly raised plateau in the middle, and as the value comes down the side of the bowl, it reaches the middle and settles up on the plateau, where it has a consistent non-zero value.

Now, for most of the history of cosmic expansion, gravity and the mass of the universe has been enough to keep expansion at a reasonably steady rate. As expansion continues, however, the mass in the cosmos becomes more and more spread out, so its gravitational effect is less and less, until it reaches a critical value at which point the HIggs field once again comes into play and drives expansion to accelerate.

The above is a feature of Alan Guth's inflationary model, which is a reasonably strong candidate for overcoming certain problems of the early universe, such as the horizon problem, an interesting problem with the standard big bang model, in which the isotropy of the cosmos is difficult to explain because of the finitude of c (the speed of light).

There are other problems to be overcome, and the Guth inflationary model is not without its own problems in terms of 'fine-tuning', which doesn't mean what creationists think it means, in that it isn't a problem of the fine-tuning of the universe, but a necessity for certain parameters to fall within a certain narrow range of values if the model is correct. There is no reason to suppose that the values of these parameters could actually be any different, but it makes a model more difficult to accept if it is restricted in this way.

I rather like Fred Hoyle's notion of a Steady State universe.

That was falsified by Hubble in the 30s.

I wonder if there is anyway to incorporate Steady State ideas into current observations?

No, there isn't. A steady-state cosmos is one which doesn't expand or contract, but remains in a 'steady state'. Our cosmos is expanding, and irrefutably so.

Something about the Big Bang just doesn't sit right with me, but that's just a gut feeling with little to back it up.

Well, as far as it goes, the big bang model, with a few modifications such as Guth's inflationary ideas, is the best model we have, but it is nothing like complete. The cosmos is definitely expanding, and it was therefore smaller in the past. That much is certain. The big problem with the big bang model as it currently stands is that it doesn't provide us with even a beginning to expansion, because it only goes back to a finite time after the beginning of expansion, namely the Planck time, which is 10^-43 seconds after the beginning. What preceded the big bang (which wasn't big, and didn't bang, incidentally) is not yet known, although we have some really solid ideas on the table. Perhaps the LHC will turn up some answers in the coming years.

I think I heard somewhere that a few cosmologists are taking a second look a Hoyle's ideas.  Do you know anything about this?

Hoyle had a lot of good ideas, but the steady state cosmos wasn't one of them. Einstein also believed in a steady state cosmos, incidentally, which is why he erroneously introduced the cosmological constant into general relativity, because he wanted something to counteract gravity to stop the cosmos from contracting under its own mass. He later called it the greatest blunder of his life. We now have a cosmological constant again, although in a much modified form, to account for the accelerated expansion.

[hackenslash]

Einstein already showed that ordinary matter is a form of energy. So it's possible that dark matter is a form of dark energy, or that both are different aspects of some still-unrevealed entity. One problem with all of this is that dark matter is apparently trying to pull the universe together whereas dark energy is apparently pushing it apart. So there still isn't a sure answer.

Bingo! Dark energy is repulsive, while dark matter is attractive. However, if you look at the role of the Higgs field, some interesting things turn up, because not only is the Higgs field postulated as a candidate for cosmic expansion, it's also postulated to be the thing that imparts mass...

[hackenslash]

Could it be that stuff actually IS "just flying apart"..? The amount of kinetic energy achieved by matter in the Bang had to be... Well, of cosmic proportions... So are we sure we are not in that phase where the bullet has not yet left the barrel, so to speak..? Where the momentum of our charge has not yet been fully countered by the ever-weakening gravitational bonds between the galaxies flying apart..?

Just speculating - that particular area of space sciences is not exactly something I've studied much.

I've toyed with that idea on a few drunken occasions...

It's certainly a possibility I haven't come across any cause to rule out, but I'm not really a physicist, and the material in this field is substantial! It always occurs to me, though, as I sober up, that it must have problems with it, or I think it would be discussed. Perhaps it's time I actually delved into it.

Actually, if Guth is right, then this idea is ruled out, or we'd be expanding at the same rate as the initial inflation or greater, and we can't be doing that, because the horizon problem requires that the rate of expansion was absolutely enormous during the early phase.