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gregtomko
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If the universe was at or near a singularity in the past, why is it not a black hole now? How can part of the universe become a black hole, and not the whole universe?
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gregtomko said:If the universe was at or near a singularity in the past, why is it not a black hole now? How can part of the universe become a black hole, and not the whole universe?
The "big bang" as far as I understand, refers to a time when the universe was very small. It then expanded. When there is a high enough concentration of mass in our universe now, it becomes a black hole. Why is it that when all the mass was concentrated at or around the "big bang" did it not just become a black hole?marcus said:Your questions don't seem to make sense.
Well the term "big bang" is misleading because it suggests an explosion from a point outwards into empty space. That is not the idea. "start of expansion" is more neutral.gregtomko said:The "big bang" as far as I understand, refers to a time when the universe was very small. ...
Mordred said:There are a couple of models that represent how this works.
The one with the best fit to data is lamdaCDM.
marcus said:High density does not by itself cause collapse to a black hole.
There is a kind of "tug-of-war" contest between the expansion rate and the density.
gregtomko said:It sounds a little funny that at the beginning, expansion was super fast. Then during most of forever, expansion was close to linear; and just now expansion is speeding up again. I know that’s what the models say, but it seems like a pibtac
Just to clarify, inflation occurs during inflaton (or vacuum energy) domination, not radiation domination. Radiation domination occurs after inflation when the inflaton decays.Mordred said:In the early universe radiation was dominated so inflation was rapid.
jety89 said:But doesn't the common origin of particles/spacetime imply some sort of singularity in the beginning?
jety89 said:OK so, initially the universe was very dense, and not neccessarily a singularity. But doesn't the common origin of particles/spacetime imply some sort of singularity in the beginning?
jety89 said:Well, theories aside, I think it still makes sense to talk about a common origin ...
I've heard things from the size of an atom to the size of a grapefruit. Folks seems to just make stuff up. BUT ... VERY small compared to the 95+billion light years across that it is now!... just how small would have been, say, the currently observable part of the universe in the Planck epoch?
... Or the universe as a whole?
jety89 said:Well, theories aside, I think it still makes sense to talk about a common origin, considering the *unchallenged* uniformity of nature: an electron is just like any other electron, anywhere, as far as anyone can tell, so we know the universe had to have a single/singular origin, we just don't have any conceivable idea how it happened beyond the Planck epoch. And, as I understand from the comments above, it needed not to be infinitesimally small at that point. So just how small would have been, say, the currently observable part of the universe in the Planck epoch? Or the universe as a whole?
phinds said:A common origin for WHAT? The universe and black holes? I don't think so.
This is shear speculation. Your assertion that all things emerged from the same singular origin because they are similar is but one possible conclusion. First, as has been painstakingly laid out in this thread -- the big bang singularity is not physical. You can imagine that there was a singularity in the past, and you can call this thing the big bang, but you wouldn't be doing science then. You'd do just as well to call it God.jety89 said:The fact that, for example, photons coming from opposite ends of the observable universe report about a remarkably self-similar universe, talks about that in the past, the predeccessors of those parts of the universe were not simply closer to each other, but were, in fact, the results of the same process, that, as it seems now, happened prior to the Planck era. You could call that a singular origin of the universe.
Now, a singular origin implies finiteness, so I think the universe we live in is finite, although very-very big.
jety89 said:I think what we can know about the BB, is that it was the singular origin of the universe.
jety89 said:The fact that, for example, photons coming from opposite ends of the observable universe report about a remarkably self-similar universe, talks about that in the past, the predeccessors of those parts of the universe were not simply closer to each other, but were, in fact, the results of the same process, that, as it seems now, happened prior to the Planck era.
jety89 said:You could call that a singular origin of the universe. Now, a singular origin implies finiteness, so I think the universe we live in is finite, although very-very big.
Nobody doing serious science refers to the big bang as an "explosion". And it is *not* merely philosophy to discuss the nature of the early moments of the expansion -- we have actual data on these points! Observations of the cosmos reveal a uniform, isotropic expansion of space: this is supportive of the notion that even back to the earliest times, expansion was occurring uniformly throughout space and was not localized. So while for sure we don't know the mechanism that started the expansion, we have a pretty good idea that it was not a localized event.tia89 said:I also DO call that the singular origin of the Universe (read my other posts). But there are interpretations claiming that the Big Bang is a simultaneous "explosion" happening in all the points of an infinite space-time rather than an "explosion" from a localized "single point". But this is all philosophy, not physics, and no one is right or wrong... we simply do NOT know.
bapowell said:And it is *not* merely philosophy to discuss the nature of the early moments of the expansion -- we have actual data on these points!
jety89 said:During the Planck epoch, as I understand from the previous comments, all that existed, was a very dense sea of "something". So what can we tell with some level of certainty about that "something" that made up this "soup"?
During the Planck epoch, we don't know what particles (or more generally "degrees of freedom") are relevant. We do, however, generically expect that there will be quantum mechanical manifestations of gravity that will govern the dynamics in some way -- we need a complete quantum theory of gravity in order to understand the specifics, but much progress has been and we are beginning to get a sense of the broad features of this era. Unfortunately, the particle spectrum is still uncertain and likely depends on the details of this as-yet unknown theory.During the Planck epoch, as I understand from the previous comments, all that existed, was a very dense sea of "something". So what can we tell with some level of certainty about that "something" that made up this "soup"?
The Big Bang theory is a scientific model that explains the origin and evolution of the universe. It states that the universe began as a singularity, a point of infinite density and temperature, and has been expanding and cooling ever since.
A black hole is a region of space with a gravitational pull so strong that nothing, including light, can escape from it. It is formed when a massive star collapses under its own gravity.
The Big Bang theory explains the beginning of the universe, while black holes are a result of the universe's evolution. Black holes can grow in size and mass over time, and are thought to play a key role in the formation of galaxies.
No, we cannot observe the Big Bang directly as it occurred about 13.8 billion years ago. However, we can observe the cosmic microwave background radiation, which is considered the leftover energy from the Big Bang. We can also indirectly observe black holes through their effects on surrounding matter and light.
The Big Bang theory is more widely accepted by the scientific community as it has more evidence and observations supporting it. However, the existence of black holes is also well-supported by evidence, including gravitational waves detected by LIGO. Both theories are important in understanding the history and workings of the universe.