Speed of Light question

[Kemrain]

Some older models of the universe predicted a collapse, in which case all the black holes would get closer and closer and absorb each other. These models weren't supported by observations, though.

The 'Big Crunch' theory. I remember when that was popular. Those were the days.

Nope, physicists are predicting now, much like fuindordm says, that the universe will eventually be nothing but black holes moving away from eachother forever. The universe will 'end' with a quiet, frigid shudder, not a cataclysmic bang. For some reason, this seems more depressing to people than the Big Crunch theory. Strange.

I'm wondering why people aren't taking this to it's next extreme, though. Wouldn't those black holes (I wish we'd stop calling them holes, it would make things so much less confusing to call them Darkstars) emit most of their mass as Hawking radiation eventually? That would leave them as masses too small to be black holes, and fill the universe with hawking radiation, wouldn't it?

Umbran and fuindordm, I think you're doing a fine job explaining these things. I understand them MUCH better now than I had before, and your consice explanations are the culprit. Don't think you're doing a bad job, because this highschool grad with no formal physics training at all has gained a much deeper understanding of the nature of gravity and black holes because of you. Thanks a lot, this stuff is wicked interesting.

What I'm interested in at the moment is this crazy String Theory stuff. Care to give that one a deeper try?

- Kemrain the Psysicophyle.

 

[fuindordm]

I'm wondering why people aren't taking this to it's next extreme, though. Wouldn't those black holes (I wish we'd stop calling them holes, it would make things so much less confusing to call them Darkstars) emit most of their mass as Hawking radiation eventually? That would leave them as masses too small to be black holes, and fill the universe with hawking radiation, wouldn't it?

What I'm interested in at the moment is this crazy String Theory stuff. Care to give that one a deeper try?

- Kemrain the Psysicophyle.

They have: check out this book by a professor from my alma mater:

The Five Ages of the Universe: Inside the Physics of Eternity by Fred Adams

As for string theory, I'm already out of my depth. The Green book is a great one.

And thanks for the kind words.

Ben

 

[Umbran]

The 'Big Crunch' theory. I remember when that was popular. Those were the days.

Well, the "Big Crunch" and the "Heat Death" universes are not really separate theories. They are merely different possibilities from the same model. If there's enough mass in the Universe, it'll crunch. If there isn't enough mass, it never stops expanding, and eventually every bit of potential energy in the Universe gets ground down into waste heat, and nothing can ever happen again.. The Big Crunch is out of favor because we dcan't find evidence of enough mass for it.

For some reason, this seems more depressing to people than the Big Crunch theory. Strange.

Not at all strange - because buried in the Big Crunch is the possibility of the Oscillating universe: another Bang that follows the Crunch. That gives at least the possibility that something might come after, and the possibility of something is usually less depressing than the surety of nothing.

I'm wondering why people aren't taking this to it's next extreme, though. Wouldn't those black holes (I wish we'd stop calling them holes, it would make things so much less confusing to call them Darkstars) emit most of their mass as Hawking radiation eventually? That would leave them as masses too small to be black holes, and fill the universe with hawking radiation, wouldn't it?

"Holes" really is a better description, to my eye. You can drop a thing into a hole, and expect to never get it back again. It isn't just a star that is dark.

Yes, the extreme you mention is recognized - eventually, all the black holes would emit their mass as Hawking radiation. And all the particles that could decay will eventually do so. And you're left with a Universe of elementary particles spread infinitely thin, all at just that minute smidge above absolute zero temperature required by the uncertainty principle.

What I'm interested in at the moment is this crazy String Theory stuff. Care to give that one a deeper try?

Well, in many ways, there isn't much to tell.

There are a number of different String Theories. They all have in common the idea that the universe isn't made up of point particles, so much as little knots of "string". And the different ways they are twisted up and vibrate tell you about the particles the strings are.

The there's three things about string theories:

1)They are, mathematically speaking, horrendously complicated. In some ways they have elegant design, but actually squeezing real-world results out of them for all but the simplest things is quite a chore.

2)None of them have made any testable predictions yet. Either the things they do predict are outside the energy range we can easily reach, or they just don't predict anything different at all. Perhaps, with time, we will get solid tests of some of them. Until then, though, they're more methemtical curiosities.

3)Rather quickly out of quantum mechanics we got useful things - like modern electronics, and nuclear power. The energy ranges required to see String Theory effects are quite high, far above what's seen in the normal home. It may be a while before we see any impact of these in our everyday lives, if ever.

 

[ssampier]

And these posts certainly wouldn't be my choice for trying to teach this stuff, either. Remember that entire college courses and books can be done on this topic. Addressing questions in a haphazard way, without any visual aids, is certianly going to be disjointed.

You want a primer that's good for folks without the background? Stephen Hawking's A Brief History of Time covers the basics. I think it has all of one equation in it. Theory developed after that book was written is coverred in The Universe in a Nutshell.

Thanks. I don't feel so dumb. Hard science has never been my thing.

 

[fuindordm]

Well, the "Big Crunch" and the "Heat Death" universes are not really separate theories. They are merely different possibilities from the same model. If there's enough mass in the Universe, it'll crunch. If there isn't enough mass, it never stops expanding, and eventually every bit of potential energy in the Universe gets ground down into waste heat, and nothing can ever happen again.. The Big Crunch is out of favor because we dcan't find evidence of enough mass for it.

To elaborate a bit:

The current cosmological theory is derived from General Relativity and the assumption that on large scales the universe can be treated as a perfect fluid of some variety, or as a mixture of perfect fluids. We're talking about scales larger than the average distance between galaxies here, so normal matter (most of which is in galaxies) is treated as a collisionless, pressureless gas (because galaxies almost never come near enough to each other to interact). The eventual fate of the universe, as Umbran points out, depends only on what it's made out of--the theory describing its evolution is the same in any case.

At early times, most of the mass/energy density in the universe was in the form of radiation.
At later times, most of the mass/energy density was in the form of neutral matter.

Twenty years ago, the only question was how much matter there was in the universe--since not all of it is in luminous stars, it's harder to add up than the radiation. People have been trying to measure the amount of dark matter for a long time, generally coming up with anywhere from 30% to 90% of the amount needed to stop the expansion of the universe. No one has ever come up with enough matter to cause a Big Crunch, and only the die-hard theorists have come up with enough matter to slow the expansion to a halt. (A universe with exactly enough matter and energy to just barely stop expanding is slightly easier to explain, as its density is a stable number--other models of the universe have an energy density that evolves with time, and a certain amount of fine-tuning is required in the initial conditions to get a universe old enough to create stars.) So the Big Crunch was never really that viable as a theory.

Aside from matter and energy, there's a third medium that can drive the evolution of the universe. This is 'vacuum energy', the energy density you find even in completely empty space due to the creation and annihilation of particle pairs according to the uncertainty principle.

In the framework of General Relativity, this can be described with a single number: the cosmological constant. In the framework of quantum field theory, it's much harder to quantify and no one yet has come up with a calculation of vacuum energy density that arrives at a reasonable number. According to theory, the vacuum energy should either be zero or some enormously high value that would make the universe expand into nothingness in an unreasonably short time. So for a long time the cosmological constant was assumed to be zero--why complicate the theory? Recent observations of the cosmic microwave background, the distribution of galaxies in the universe, and distant supernova, however, have shown pretty conclusively that the cosmological constant is not zero.

Since vacuum energy density remains constant as volume increases, it exerts a different kind of pressure on space-time than radiation or matter. The existence of vacuum energy (also called dark energy or quintessence) would eventually drive the universe into a phase of exponential expansion, so the empty universe is coming much sooner than we think. But it's not all bad--this expansion is only of the space between gravitationally bound systems. By the time it happens, our galaxy should be safely tucked away in the Virgo cluster of galaxies, still giving us several thousand galaxies to play in.

But again, it's really all one theory. It's just a matter of plugging in different amounts of matter, radiation, and cosmological constant, and seeing how the mixed fluid evolves according to general relativity.

Ben

 

[Umbran]

Since vacuum energy density remains constant as volume increases...

Or, so we hope. It is important to note that for most "constants", the fact that it is constant (in either space or time) is usually only a guess. There's the occasional theory that plays with the idea that they aren't...

That may seem a bit at odds with the discussion of the speed of light being constant. But that's one of the better known and measured constants - the consequences of having it be non-constant have been well thought through. As I understand it, there's really no evidence for it varying in space, and only a little wiggle room for it varying in time.

But, it is hard enough to just measure the cosmological constant, so it is nigh impossible to prove that it really is constant.

 

[fuindordm]

Or, so we hope. It is important to note that for most "constants", the fact that it is constant (in either space or time) is usually only a guess. There's the occasional theory that plays with the idea that they aren't...

That may seem a bit at odds with the discussion of the speed of light being constant. But that's one of the better known and measured constants - the consequences of having it be non-constant have been well thought through. As I understand it, there's really no evidence for it varying in space, and only a little wiggle room for it varying in time.

But, it is hard enough to just measure the cosmological constant, so it is nigh impossible to prove that it really is constant.

Well, the cosmological constant, if it exists, does have to be constant in time, otherwise it wouldn't fit into the equation of general relativity correctly. This doesn't prevent us from having a vacuum energy density that evolves with time, however. A vacuum energy can behave like a cosmological constant, but it can also be more complicated--it goes elsewhere in the equation.

But I was really talking about an energy density that was constant in space. If the 'dark energy' turns out to be due to the creation and annihilation of particles in vacuum, then this is a process that should be going on in the same way everywhere that there isn't matter or significant curvature of space-time. So if you compare two boxes of vacuum, one with twice the volume of the other, the bigger box will contain twice as much vacuum energy as the smaller.

When you treat vacuum energy as a perfect fluid, this leads to a very different equation of state than what you get from matter or radiation, whose density always decreases with increasing volume.

Ben

 

[Harmon]

Flipping channels on Wednesday or Thursday came across the middle of a show on Discovery, one of the guys they were interviewing said something like- "there isn't enough (matter/material/energy) in the universe to justify its creation."

I kinda scratched my head, and thought about black holes for a moment- "if what ever amount that isn't here has already been taken into black holes then would that mean that he's really got no clue about the amount of (matter/material/energy) in the universe?"

What about time? I mean we are looking across a minunum of 5+ yrs to the nearest star to get information, and most of the cool universe stuff is decades, hundreds, and thousands of years older then that, so how can we make an educated understanding of the universe based on reactions and forces that are so much out of date.

Umbran & Fuindordm- thanks for picking this up. I love to read and watch this sort of converstaion, makes me feel more smarter then me am

 

[the Jester]

Though we can't see what's in a black hole, we can measure its mass indirectly (through its effects on the stuff around it).

 

[Alzrius]

Correct me if I'm wrong, but I thought the dark matter theory had recently been debunked?