https://www.msn.com/en-us/news/tech...pc=U531&cvid=272cb184de9c48fbbd3b321120e37dac
It's more or less true in the current epoch, but not exactly so. It'll be true briefly at some point in the future for one instant before it starts becoming less accurate again. It doesn't have anything to do with black holes despite Kaku's implication. Black hole spacetimes don't look much like FLRW spacetimes.Ivan Seeking said:
I'm pretty sure I hear Carl Sagan (on Cosmos) say, "If you want to know what it looks like inside of a black hole, look around." But I could be misremembering.Ivan Seeking said:
Well, there isn't much exciting to see in a black hole. Infalling radiation may be blue or red shifted depending on your velocity, but that's about it. The accretion disc will probably be an impressive sight above you, but everywhere else will be pretty much black and unexciting. There's nothing particularly special about the interior of a black hole in GR, until you get to the point where tidal forces rip you apart. Even then l, there's nothing to see particularly, apart from the mess.gmax137 said:
Does that include dark matter and dark energy?Ibix said:
Yes. You can have a simpler model of the universe without dark energy (or does it have to be matter dominated? Not sure without double checking) and then you find that the radius of the observable universe is exactly the Schwarzschild radius. With the mix we have, the radius varies a bit compared to the Schwarzschild radius.Ivan Seeking said:
It isn't much of a coincidence. ##GM/c^2## is a natural length scale for gravitational phenomena when you have mass ##M##, so it's not very surprising that the Schwarzschild radius and the radius of the observable universe are simple multiples of it. That they both happen to be twice it is a bit of a coincidence, but that's about it. Almost any maths you do in GR will spit out the Schwarzschild radius or a simple multiple of it somewhere.Ivan Seeking said:
No room! I'm curiously reading that.berkeman said:(Time to split this conversation off into a new thread, or do you two want to just get a room?)
Well you have the superpowers for el-splitto...fresh_42 said:
I hope we didn't kill that cat.berkeman said:
There is no consensus on how you even define the total energy of the universe. We only know how to do it in asymptotically flat spacetimes, and cosmological spacetimes aren't asymptotically flat. So, um... maybe?Ivan Seeking said:
Dark energy density remains constant at all times, which means it becomes more important over time as the density of everything else falls. It's tempting to try to equate it to the vacuum energy density of quantum field theory, but the best numbers we can come up with for that differ by about 120 orders of magnitude from the measured value... So we don't really know much about it.Ivan Seeking said:
The cat that lays the golden eggs?fresh_42 said:
Ibix said:
Ibix said:
That is REALLY going to bother me. LOL! I have a hard time buying the argument this is coincidental. I could believe it is circular reasoning somehow; in fact, I would bet on it! But that it accidently matches, exactly? Hmmm.Ibix said:
If one accepts that "dark energy" is really just the cosmological constant, all such nonsense falls into the dustbin where it belongs. (Sigh.)Ivan Seeking said:
No. Michio Kaku is notorious for making claims in pop science contexts that he knows he would never get away with if other physicists were around to challenge them.Ivan Seeking said:
I don't know that we are looking for a "seed BH". However, it is theoretically possible to have a black hole form from dark matter. It's less probable than with regular matter because it's pretty much collisionless, so doesn't collide, lose energy, slow down, and collapse the way normal matter does. When it could happen is in the early universe when densities were high, but everything is more uniform so finding an overdense region that doesn't contain any normal matter is really improbable. So it's possible to have a black hole that formed from pure dark matter, but the conditions for it to happen are improbable, shading rapidly towards impossible if you mean anything larger than a microscopic black hole.fresh_42 said:
FLRW spacetimes model the universe as the same everywhere. So the first question is if it's infinite. If it is then its total energy is probably going to be undefined.Ivan Seeking said:
Dimensional analysis will tell you that any natural length scale you get out of GR is going to be a dimensionless multiple of ##GM/c^2##. There are no other relevant constants to play with and that's the only mix of ##G##, ##M## and ##c## with a dimension of length. So it's only the 2 that's a coincidence. And it's the same basic equations (Einstein's field equations) in both cases, and spherically symmetric solutions in both cases. So it's not enormously surprising to me that the multiple is the same, although the calculations are for different things and proceed in quite different ways from there.Ivan Seeking said:
They don't, always.Vanadium 50 said:
Neither do the two cosmological calculations you are comparing.Ivan Seeking said:
Peacock in “Cosmological Physics” calls it an unlimited reservoir of energy, just from my memory, I haven’t the book with me. Constant vacuum energy density and increasing size of the universe means increasing vacuum energy, whereby all other energy densities are decreasing at a known rate.Ivan Seeking said:
Because for it to be true, there would have to be some invariant that corresponds to "the total energy of the universe". And there isn't. The quantity that you intuitively think of as "increasing total energy" because of dark energy is not an invariant; it requires a specific coordinate choice to even be defined.Ivan Seeking said:
Nothing "matches exactly". That's the point. You can wave your hands and find an "M" for the universe that sort of matches, but that "M" is not an invariant any more than the "total energy of the universe" is an invariant. You are dealing with a completely different spacetime geometry from the spacetime geometry of a black hole and the "M" of a black hole has a completely different meaning--in fact a black hole spacetime is vacuum, there is no matter present.Ivan Seeking said:
This article is not at all reliable. At one point it says the radius of the entire universe (not observable, entire) is 23 trillion light years across, but at another point it says our universe is open, which means it would be spatially infinite. (Our actual best current model says our universe is spatially flat, which is different from both.)Ivan Seeking said:
The Schwarzschild radius is the radius of a sphere such that, if all the mass of an object were to be compressed within that sphere, the escape velocity from the surface would equal the speed of light. It is a critical radius for black holes, beyond which nothing, not even light, can escape the gravitational pull.
The Schwarzschild radius of the Universe can be calculated using the formula \( R_s = \frac{2GM}{c^2} \), where \( G \) is the gravitational constant, \( M \) is the mass of the Universe, and \( c \) is the speed of light. By estimating the total mass of the Universe, one can determine its Schwarzschild radius.
In cosmology, the Schwarzschild radius of the Universe provides insight into the conditions under which the Universe could collapse into a black hole. It helps in understanding the balance between the Universe's mass and its expansion, and it can give clues about the ultimate fate of the Universe.
Current observations suggest that the Universe is not within its own Schwarzschild radius. This means that the Universe is not a black hole, as it is expanding and has not collapsed into a singularity. The actual radius of the observable Universe is much larger than its Schwarzschild radius.
If the Universe were within its Schwarzschild radius, it would imply that the Universe is a black hole. This would mean that all matter and light would be trapped within this radius, leading to a gravitational collapse. However, our current understanding of cosmology and observations indicate that the Universe is expanding, not collapsing.