Gringo

I didn't say that he took the idea seriously, but just that he raised this strange possibility in an oscillating universe in one of his musings.

Cosmos p. 215

[ November 16, 2002: Message edited by: Gringo ]</p>

tronvillain

The fluctuating universe idea is definitely an interesting, but there is no reason the universe has to be the same each time. Besides, living the same life over and over is indistinguishable from just living that life once.

Seeker196

Yes, but it's comforting to know that, even though you won't realize it, one day you will get that great 11th Grade BJ again....and again...and again...

Defiant Heretic

I'm fairly certain that quintessence, dark energy, and the cosmological constant all refer to the same thing.

Kharakov

Did you ever think that matter is just falling in on itself, and that is the reason for gravity and the apparent expansion of the universe?

eh

They don't. Though they do atempt to explain the same things.

Answerer

Maybe 'dark energy' is due to the gravitional force from other universes or higher dimensions(if there are).

Gauge Boson

While we have digressed to assessing the possibility of a Big Crunch, I figure I could quickly sum up what we do know and what we don't know.

• We know:
• The Hubble constant is increasing (the expansion of the universe is accelerating).
• All luminous matter, dark matter, and radiation in the universe only total 1/3 critical density.
• The universe is within two percent of flat.
• Therefore, some form of dark energy must account for 2/3 of the energy in the universe.
• This dark energy is spacially uniform and gravitationally repulsive.
• The dark energy can cause a positive or zero cosmological constant, but not a negative one (if it is caused by vacuum energy, by far the most plausible explanation, though Ekpyrosis predicts this, too).

• What could it be?
• Cosmological constant - if the cosmological 'constant' is truly constant (except for turning on at inflation, off at its end, and back on when the universe becomes matter-dominated), the rate of expansion will increase exponentially. This theory is based on dark energy caused by vacuum energy.
• Quintessence - in this, the cosmological 'constant' is more dynamical. It is controlled by a scalar field; IOW, it behaves over time as if its energy tends toward a certain state. This scalar field controls the level of vacuum energy and hence the strength of the cosmological 'constant'. This prefferred energy state can change over time. Quintessence can increase, remain constant, decrease, or turn off. It CANNOT reverse without drastic changes to the fundamental laws of physics.
• Ekpyrosis - literally, "conflagration". This M-theory cosmology essentially says that what we percieve as the universe expanding is an increase in the space between our 'brane universe' and a 'shadow brane'. The branes could come together, resulting in an apparent collapse of our universe (though both branes would be infinite in this model). When they collide, the universes are 'reset' and can be completely different after this additional Big Bang. This requires that the separation between the branes must be at least 1/10 millimeter, and a strict upper limit of 1/2 millimeter already exists. The future of Ekpyrosis looks dim.
• Dark energy - we have very good reason to believe that this comes from vacuum energy. Vacuum energy has all of the right properties: it is gravitationally repulsive, it is spacially uniform, and it is for the most part otherwise invisible. Vacuum energy, however, is an observed fact. The basics of the phenomenon are understood well, though we don't know what counters the last little extra bit of dark energy left over in current calculations.

A universe with a cosmological constant or quintessence (or any other theory based on vacuum energy) will expand forever and will do so at an ever-increasing rate. If the flat curvature is an illusion, we live in an open universe that will expand and do so ever-faster. If the accelerating expansion is somehow an illusion, we are then at critical density, so the universe expands forever, though always slowing down. If Ekpyrosis is correct (though this now seems unlikely, except in its new formulation that says nothing about cosmological acceleration), the universe may enter another hyper-inflationary epoch or may appear to recollapse.

[ November 18, 2002: Message edited by: Gauge Boson ]</p>

Gauge Boson

There are a few problems with this. First, a collapse of the matter in the universe would look just like that: a collapse. It certainly would not look like expansion. Everything would be blue-shifted, but instead everything (outside of our local group) is red-shifted.

As for the gravity part.... I don't know where to start with that one. The only meaning I can derive from that statement is a claim that 'things are falling toward eachother, so it looks like they are being pulled together, so that explains gravity'. It is very simple to show that this is wrong. If that were the case, everything would continue on its original path, unaffected by gravity (which would not exist). So if two objects were not on a direct collision course, they will not collide. So Comet S-L 9 rules that out. So does throwing a baseball in the air and seeing it come back down. If the appearance of gravity is only from objects falling together, then a baseball thrown into the air would not be pulled down by anything. Without actual gravity, you, everyone else, the oceans, and the atmosphere would all be flung from Earth. If, OTOH, you are saying 'gravity causes things to fall together, so things falling together must cause gravity', you are commiting a logical fallacy (I can't remember the name of this one). A causes B does not mean that B causes A. Example: my alarm clock causes me to get up, but me waking up does not cause my alarm clock to go off.

Kharakov

Not exactly what I was getting at.

What if the appearance of expansion is caused by matter shrinking-- everything appears to be receding because everything is getting smaller. If matter takes up less space, it would be farther away to the closest point of contact with the matter if it was centered at the same point.

If matter shrinks, and light stays the same size (wavelength), than lights wavelength would appear to be longer (redshift). Lets say that the wavelength of light emitted by matter is determined by the actual size of the matter (so 15 billion years ago, light produced by hydrogen fusion would have had a longer wavelength than light produced today). The farther back in time you get, the longer the wavelength of the light (because matter was larger back then).

This is just a crazy idea of mine .