# Tag Info

25

Yes, you are absolutely right, from OUR VIEWPOINT it does. From Kip Thorne's book "Black Holes and Time Warps: Einstein's Outrageous Legacy." “Like a rock dropped from a rooftop, the star’s surface falls downward (shrinks inward) slowly at first, then more and more rapidly. Had Newton’s laws of gravity been correct, this acceleration of the implosion would ...

25

What you're describing is basically the "collapsed star" (Eng) or "frozen star" (Rus) interpretation of black holes that was common prior to the late mid-1960s. It was a mistake. Suppose you are distant and stationary relative to the black hole. You will observe infalling matter asymptotically approaching the horizon, growing ever fainter as it redshifts. ...

22

The problem with trying to form a black hole with dark matter is that dark matter can only weakly interact (if at all) with normal matter and itself, other than by gravity. This poses a problem. To get dark matter concentrated enough to form a black hole requires it to increase its (negative) gravitational binding energy without at the same time increasing ...

21

You're envisioning the Big Bang as a cosmological "explosion" in space where the universe is the resulting material expanding in outward all directions. The problem is, the universe doesn't work like that. The universe isn't a region of space that is expanding outward into another thing where you can (even hypothetically) fly out to the border and say, "...

19

Yes, here. As has already been mentioned, the big bang happened everywhere due to the fact that all of time and space was in the same spot at the point of the Big Bang. And so the big bang happened here, there and everywhere. This is also similar to saying that the universe has no centre. I have seen this phenomenon explained using two pieces of clear ...

14

We lack the precision to say that there aren't regions where there is matter without dark matter or vice-versa. But what is clear is that the ratio of dark matter to normal matter, which is (or needs to be) around 5 on average to explain the flatness of the universe, varies by orders of magnitude from place to place. The reason for this is that matter ...

13

Matter is the stuff you are made of. Antimatter is the same as matter in every way, looks the same, behaves the same, except its particles have electrical charges opposite to matter. E.g., our electrons are negatively charged, whereas a positron (an antimatter "electron") is positively charged. The positron is the "anti-particle" of the electron. When a ...

12

There are certainly people who study alternative (non-General Relativistic) theories of gravity. The most popular theories have so far been: Modified Newtonian Dynamics (MOND) - which essentially postulates that Newtonian Mechanics break down on some scale, leading to the rotation curves we see in galaxies. Tensor–vector–scalar gravity (TeVeS) - this is a ...

11

We need to think about just where the time dilation effect occurs. By then thinking about the observations from each point of view, that is the free falling object and the external observer, we can come to terms with just what is happening as opposed to what appears to be happening. The experience of time We must remember that an object moving at a certain ...

10

The interstellar medium is a multiphase medium, and you can find (some references in this lecture and in this thesis manuscript (this one is in French, but numbers are international)): the hot ionized medium (HIM) with density as low as 10^-3 cc (particles per cubic centimeter); the warm ionized medium (WIM), with density of the order of 0.03 cc; the warm ...

10

When you say "particle" candidates, I assume you're excluding MACHOs and RAMBOs. MACHOs are "dark" objects at the stellar scale like black holes, neutron stars, brown dwarfs, etc. RAMBOs are clusters of similar dark objects. MACHOs and RAMBOs are made of primarily baryonic matter (everyday stuff like protons and neutrons — electrons are not baryons but ...

9

As pointed out by Rob Jeffries, forming a black hole (BH) from dark matter (DM) is impossible (unless there is a [hypothetical] interaction by which dark-matter can lose energy that evades all detection). Accreting DM into an existing BH is still unlikely (since DM cannot lose its excess energy and angular momentum as easily as gas), but not impossible and a ...

9

The material (heavier than helium) that makes up the solar system was made in millions, if not hundreds of millions of stars that lived and died in the ~7 billion years between the formation of the Galaxy and the birth of the Sun. This material has been thoroughly mixed in the interstellar medium and so the heavier elements arise from countless individual ...

8

It's all around us. The way I explain it is that we are in the centre of the universe (I don't mean in the geocentric way as thought back in the middle ages where all is moving around us). Everywhere we look we see the past: our Sun how it was 8 minutes ago; the closer star, Alpha Centauri, as it was 4.2 years ago, the Andromeda galaxy's light took 2.5 ...

8

Yes, space is constantly being created. The new space does not hold any matter (like atoms) or dark matter. This means that the density of normal and dark matter decreases at the same rate as the volume increases. However, dark energy, which is something completely different and thought to be a property of vacuum itself, is being created with the new space, ...

8

It is simply not true that gravity can only interact with mass. Rather, any long-range spin-2 force interacts with all energy-momentum equally, and it source is the stress-energy-momentum tensor. That is one way to state the equivalence principle. Note that a massive object in its own rest frame has an associated energy $E = mc^2$, which under ordinary ...

7

That's a very complicated question! First, let's remember that Moses didn't bring the Law of Conservation of Energy down from Sinai on stone tablets -- it's something that we've observed to be true of the universe today, and which our theories use to make very accurate predictions. But this doesn't mean that it was always true, and particularly that it was ...

7

Black holes are not "made of matter". They are better described as structures of gravity/warped spacetime. However, they do grow when absorbing things with positive mass-energy. Antimatter still has positive mass-energy, so the black hole cheerfully gobbles it up. Now, exotic matter with negative energy density might indeed decrease the size of the black ...

6

It really depends what you mean by "rock". At the temperatures and pressures at the cores of stars (and at which nuclear fusion reactions are possible), "rocks" as I suspect you are thinking of, do not exist. Thermonuclear reactions do not occur because the gas is "flammable", they occur because the kinetic energies of the nuclei in the gas (at these ...

6

The logical consequence is, that an event horizon cannot form, since the first particle slows down asymptotically to zero, just before the event horizon forms (Fermat's infinite descent). The emergence of the event horizon therefore takes infinite time seen from outside. But due to Hawking radiation a black hole exists only a finite time. Hence an event ...

6

Conceptually there are several things going on here. Where does energy conservation come from? In modern understanding, energy is the Noether charge of time translation symmetry, as found by Noether's first theorem. But in general relativity, the metric is dynamical, and so in general we simply don't have any time translation symmetry. Static spacetimes do,...

6

About 70% of the baryonic matter in the universe is hydrogen, with a mean density of about $4\times 10^{-29}$ kg/m$^3$. Most of the stars that have ever been born are still alive, since an average star is only about $0.25 M_{\odot}$ and has a lifetime much longer than the age of the universe (so very little material has actually been recycled). If we ...

5

My understanding is that the Big Bang took place everywhere. That "one spot" expanded into the Universe as we see it now. There is no distinguished spot within the current Universe corresponding to the Big Bang.

5

Several wonderful yet technical answers have been given, and I cannot add anything to those very nice answers that explain why it is not useful to think black holes get "frozen" at their event horizons. But I can give an answer with a more essentially useful philosophical perspective, which is that the central lesson of relativity is that reality involves a ...

5

Thought provoking cosmologists! I'm uber late to this discussion as I see it has been ongoing for literally years and don't know if there is still anyone monitoring this thread, but here' goes. I studied astrophysics at UC Berkeley in the late 80's so perhaps my info is a little dated, upfront apologies if so. I spent a lot of time thinking about this ...

5

Black holes are the result of mass so concentrated that gravity does not let anything out, including light. Just about the only things we know about dark matter is that it has mass and seems to only interact with ordinary matter through gravity. Since we do not know the physics of dark matter at all, it is impossible to say what processes might concentrate ...

5

What's going on here is that you have been misled into thinking the ring-like structure has anything to do with the accretion disk. It doesn't, or at least only indirectly. The disk is referred to as geometrically thick, but optically thin (see sections 1 and 2 of paper V issued by the Event Horizon Telescope collaboration on 10 April 2019). This is ...

4

What we call "gravity" is really just the distortion of spacetime. If spacetime was completely straight, there would be zero gravity. But the existence of a massive body is one of the things that can distort spacetime, so that "straight" lines are not straight anymore. We perceive that distortion as "gravity". It is the distortion that actually generates the ...

4

It is unfortunate that the usual poor journalism labels the growth of the black hole as "inexplicable" and then further down in the article refers to some possible explanations. The basic problem is a growth timescale one. Radiation pressure introduces a negative feedback, such that there is a "theoretical" maximum for spherical accretion called the ...

4

To answer your second question first: Yes, antimatter does exist in the same space as matter. In fact, the universe creates antimatter (and an equal amount of matter) every day as a matter of course in events like lightning strikes and supernovae, and even in certain nuclear decays. Humans create it in particle accelerators for research and for commercial/...

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