Genady

53 minutes ago, Danijel Gorupec said:

So, without the dark energy nuisance, we should be able to estimate the portion of uniformly distributed dark mater. But because of the poorly understood dark energy, I guess we cannot do much of an estimation?

Dark matter and dark energy are separable in a variety of ways. Dark matter attracts, dark energy repulses. Dark matter density falls off with a cube of expansion, dark energy density doesn't change with expansion. Dark matter non-uniformities accelerate clamping of regular matter, dark energy doesn't have such effect. ... 

14 minutes ago, Danijel Gorupec said:

Why? It seems the same to me - you can have positive-mass around galaxies, or you can have negative-mass everywhere else. How could you tell the difference?

By the Newton's theorem, a particle inside an empty massive sphere doesn't feel any gravity from the sphere. It holds the same for positive and for negative gravity. Neither positive nor negative mass homogeneously distributed outside galaxy have any gravitational effect on the galaxy.

1 hour ago, iNow said:

More precisely, writers of headlines and random people like you enjoy claiming that scientists are easily shocked. 

Further conversation should remain focused on the dinosaur egg as that’s the actual thread topic. 

Sorry for the misunderstanding. I was kidding. Such headlines, I guess, are just click baits. Thought about it because the thread topic mentions impressing scientists. I don't enjoy claiming that scientists are easily shocked but rather enjoy making fun of popular science reports.

Evidently, scientists are easily shocked:

Scientists Shocked by Discovery of Enormous, Healthy Coral (futurism.com)

Climate scientists shocked by scale of floods in Germany | Flooding | The Guardian

Scientists 'shocked' by high levels of microplastic pollution in London's Thames (nbcnews.com)

Scientists shocked by Arctic permafrost thawing 70 years sooner than predicted | Climate crisis | The Guardian

Scientists shocked by mysterious deaths of ancient trees - BBC News

Scientists shocked to discover how much lightning may clean the atmosphere | CBC Radio

Scientists Shocked By Rare, Giant Sunfish Washed Up On California Beach : NPR

Italian scientists shocked by earthquake devastation | Nature

Scientists ‘shocked’ to find life in extreme depths under Antarctic ice - National | Globalnews.ca

Scientists 'shocked' after second coral bleaching at Great Barrier Reef in two years - CNN

... the list goes on and on.

A friend has sent this question to me. I have it solved with linear algebra (and some hand waving). Can you find a shorter way to the answer? (It's not a homework, not mine anyway. My homework times long gone.)

A gardener collected 17 apples. He finds that each time he
removes an apple from his harvest, he can share the remaining fruit
in two piles of equal weight, each containing 8 apples. Show that
all apples are the same weight.

On 11/10/2021 at 4:12 AM, Intoscience said:

With modern understanding of the quantum era, do we not describe space & time in discrete quantities ... ?

No, position and time are described by real numbers in QM.

1 minute ago, swansont said:

But then what of an observer some distance away? Everything must accelerate toward them, because the choice of the origin is arbitrary. Which can’t be true unless the acceleration is zero.

It is true. For observer 1, a particle P is at rest and a particle Q accelerates toward it. For observer 2, Q is at rest and P accelerates toward it.

Susskind answers exactly this question in the lecture that I've linked in the post above yours.

1 hour ago, Genady said:

The center is just an origin of coordinates. Of course, the premise is homogeneity and isotropy of the entire space. For any coordinate system, a particle in its origin does not move anywhere, but all other particles accelerate toward it. The same as in the Hubble expansion.

An easy derivation, with answers to the audience's questions relevant to the above discussion, can be found here, starting about 30 minutes into the lecture: 

1 minute ago, swansont said:

This is contradictory. If the accelerations cancel, there is net scceleration.

We don’t have a sphere, and there is no center. The premise is we have mass uniformly distributed over all space.

 

The center is just an origin of coordinates. Of course, the premise is homogeneity and isotropy of the entire space. For any coordinate system, a particle in its origin does not move anywhere, but all other particles accelerate toward it. The same as in the Hubble expansion.

47 minutes ago, swansont said:

How? It would all cancel. 

Each particle is attracted equally from all directions, and these will cancel indeed. But, each particle attracts all other particles and they will all accelerate toward it. This is so for all particles and thus all particles accelerate toward each other. So, the entire thing homogenously and isotropically contracts, or slows its expansion.

For a bit more precise treatment, take any particle as a center and consider particles on a sphere of radius R around it. Each particle on the sphere, according to the old Newton's theorem that holds in GR as well, is attracted to the center as if the mass of the entire ball of radius R is in the center, and effect of each larger sphere on it is 0. This holds for any point picked as a center.

The fully precise result in GR follows from increasing mass density in Friedman equation.

31 minutes ago, swansont said:

Perhaps not. It wouldn’t cause ... any net gravitational attraction.

Yes, it would cause a net gravitational attraction and that would cause slowing of the universe expansion.

On 12/23/2021 at 10:34 AM, stephaneww said:

The Casimir effect having been proved experimentally as effect of the vacuum energy

Not proved, according to this:

"In discussions of the cosmological constant, the Casimir effect is often invoked as decisive evidence that the zero-point energies of quantum fields are “real.” On the contrary, Casimir effects can be formulated and Casimir forces can be computed without reference to zero-point energies. They are relativistic, quantum forces between charges and currents.

…

I have presented an argument that the experimental confirmation of the Casimir effect does not establish the reality of zero-point fluctuations. Casimir forces can be
calculated without reference to the vacuum ... . The vacuum-to-vacuum graphs (See Fig. 1) that define the zero-point energy do not enter the calculation of the Casimir force, which instead only involves graphs with external lines. So the concept of zero-point fluctuations is a heuristic and calculational aid in the description of the Casimir effect, but not a necessity."

Casimir effect and the quantum vacuum
R. L. Jaffe
Phys. Rev. D 72, 021301(R) – Published 12 July 2005

27 minutes ago, Danijel Gorupec said:

Also, if DM is perfectly uniformly distributed, would we at all be able to detect it - could it be that there is much more of it around, but we only detect so much of it because it is not perfectly uniform?

If there were much more of it around, it would affect the cosmological expansion. Perhaps it could be detected this way.

6 minutes ago, swansont said:

AFAIK that’s why. If the only channel is gravitational radiation, then the dissipation is very, very weak. All collisions would be basically elastic.

The distribution of the DM in space now is very much non-uniform. Since it doesn't clump, was its distribution as non-uniform soon after the BB?

19 hours ago, swansont said:

If it could interact electromagnetically it would emit thermal EM radiation.

Is it necessarily so? If it could interact electromagnetically by completely reflecting an EM radiation it wouldn't emit a thermal EM radiation, would it?

On 12/21/2021 at 3:56 PM, beecee said:

It is thought by many to be some unknown form of non baryonic matter, that interacts only via gravity, and that is evidenced by the bullet cluster anomaly....

Many think that it interacts not only via gravity but also weakly, i.e. made of WIMPs.

30 minutes ago, swansont said:

Part of the reason it’s called dark is because it doesn’t interact electromagnetically, and is thus not visible 

I don't know why it's called dark, but not interacting electromagnetically makes it rather transparent than dark.

We also know that it was there by the time of "recombination", about 380 000 years after the Big Bang.

We know where it is and how much of it is there. We don't know what it is.

Nice. Except the very last statement. That is a kind of "wishful thinking" fallacy.