The Expanding Universe

[oxmixmudd]

No... that is a theoretical consequence of a form of hyperaccelerating expansion. It is not a prediction or an expectation of any physics we know of at present. It isn't even a consequence of indefinite exponential expansion (as occurred in the inflationary epoch).

The wikipedia article doesn't really explain this well. It describes the "Big Rip" as a "hypothesis", but a better word would be "hypothetical". The original Big Rip paper was not a prediction or hypothesis about our universe; but a hypothetical about our universe for an unusual value of one of the expansion parameters.

There's no good reason to think that the equation of state has ω < -1. Observational data can't formally rule it out, but that's not the same thing. Conventional dark energy has equation of state ω = -1, which is pure inflationary expansion; the LCDM models (dark matter and dark energy) would predict ω > -1, and approaching -1 in the limit of eternity.

Cheers -- sylas

Thanks Sylas.

I should have mentioned this as a possible outcome, not the expected outcome but to be fair I was thinking about it incorrectly, i.e. as having stronger support than it does. The "Hubble Site" web page breaks the possible consequences of expansion into 3 primary scenarios "Big Crunch" (ruled out by the observations implying an acceleration in expansion), "Big Chill", and "Big Rip".

fate_of_the_universe


Jim

[RBerman]

Yes! Exactly. The inflationary epoch is considered to be an time during which the expansion proceeded exponentially in precisely the way you suggest. This is the case in which the hubble constant (a misnomer if ever there was one) actually *is* constant. With a constant hubble constant, receding galaxies that are moving further and further away must also be accelerating away, since their speed is proportional to their distance. In THAT case, any galaxy receding at the speed of light or greater will be invisible; the photons from the galaxy cannot reach us, but will be always receding as well.

Of course, the inflationary hypothesis also proposes that the expoential expansion persisted for a short and finite period of time. Once inflation stopped, the conditions of invisibility are removed and light can, once again, eventually cross the furthest reaches of space even though (initially) receding at greater than light speed.

Of course, there's another wrinkle. It was discovered a couple of decades ago that the expansion rate of the universe is accelerating. This suggests we are actually now in another inflationary epoch. In the conventional LCDM models, the simplest models fitting observations for the present epoch, the accelerating expansion results in a "horizon", and as time passes more and more galaxies will recede to the point of invisiblity, in the sense that photons can never cross the reaches of space to let us see them.

One huge irony of this model is that it implies a universe where, billions and billions of years hence, astronomers living at that time will not be able to see distant galaxies, and quite likely will simply not have observational data that would let them discover that the universe is expanding! They would see a few local gravitationally bound galaxies, with no way to ever discover the rest of the universe that is expanding away from them.

What is thought to cause the changes between these epochs, so that sometimes the universe is expanding exponentially, and sometimes it's not? I would have expected whatever force was causing the Hubble value to decrease would continue to do so until it became negative, and space itself began to contract. But if I understand you properly, it's thought that the value first was positive, and then went negative, but is now positive again. That suggests it's a function of some other underlying process, I would think.

[sylas]

What is thought to cause the changes between these epochs, so that sometimes the universe is expanding exponentially, and sometimes it's not? I would have expected whatever force was causing the Hubble value to decrease would continue to do so until it became negative, and space itself began to contract. But if I understand you properly, it's thought that the value first was positive, and then went negative, but is now positive again. That suggests it's a function of some other underlying process, I would think.

The hypothesis of an inflationary period in the very early universe is associated with the decay of a scalar field, the "inflaton". I don't even begin to understand this. There are a whole range of ideas for the nature of the field and of the decay process.

I have a better feel for the current situation. The simplest model for the universe at present is the LCDM model, or ΛCDM model. This means "Lambda Constant Dark Matter". In this model, the make up of the universe is radiation, matter, and "dark energy". It is comparatively common to see introductions to the cosmology that speak of how the universe is mostly dark energy, which is an energy associated with empty space (energy of the vacuum, represented by the constant Λ), and then matter; except that most of the matter is "dark matter" which has not yet been identified and has been detected only through its large scale gravitational effects.

In a universe filled with dark energy, you get pure inflationary expansion; or exponential expansion. This is essentially because vacuum is not diluted as space expands; you still get the same constant energy per unit volume of space.

In a universe filled with matter (dark matter, or conventional matter), you get a decelerating expansion. This because as space expands, matter is spread out and diluted. You get less matter per unit volume as space expands.

In the LCDM model, the expansion of the universe is based on a combination of vacuum energy and matter. And that mean, as expansion progressed, the dark energy contribution progressively increases.

Here's a NASA page which gives a fairly accessible description: Dark energy, dark matter. Here's an extract:

By fitting a theoretical model of the composition of the Universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~68% dark energy, ~27% dark matter, ~5% normal matter.

As time passes, the percentage dark energy continues to increase, while the percentage of matter continues to decrease. One way to think about this might be that as the universe expands and thins out, you get more empty space and less matter. The gravitational attraction of matter (which tends to pull things together and to slow expansion rates) becomes less and less able to counteract the pressure of vacuum energy (which tends to pull things apart).

The turning point, in this model, would be about 8 billion years ago; that was when expansion changed over from deceleration to acceleration. The change was gradual; not sudden and not involving any sudden change in the forces involved.



BTW, the numbers on that page may be a little out of date. More recent estimates I think give a slightly younger universe (about 13.7 billion years rather than the 15 billion mentioned on the page) and slightly more dark energy (about 72% rather than 68%)

However; watch this space. Cosmology is still a long way from being well understood; and is still a rapidly developing field of science.

Cheers -- sylas