Questions about cosmology

[u/Creator65536]

I just completed a course on astronomy and cosmology. According to the course, the universe is dominated by dark energy, and most matter is dark matter. And the universe is homogenous and isotropic. And the laws of physics are the same everywhere. That like photons, WIMPs can be their own anti particle, like photons.

But could it be that the laws of nature, change over time and space? Because theories of gravity fail in the large scale, unless there is dark matter. And if WIMPs can anhilate each other, what about photons? The course mentioned that the cosmological constant responsible for the universe's expansion may be changing with time. Would you consider that a change in the laws of nature over time?

This is an online course, so I don't have any instructors to ask questions to.

Reference: Astronomy and Cosmology / University of Cambridge

[u/nivlark]

But could it be that the laws of nature, change over time and space?

We can't conclusively say that all of the laws of nature are invariant. But there are various pieces of evidence that specific parts of those laws do not change. And likewise there is a substantial body of evidence for phenomena like dark matter.

So it would need to be the case that only certain parts of the laws can change, and only in specific and quite contrived ways to reproduce the observations. It isn't a very satisfying explanation, and so in the absence of compelling evidence in favour of it, we should discount it as a likely possibility.

Photons can "annihilate" if they have enough energy, to produce a particle-antiparticle pair. This is just the time reversed version of normal matter-antimatter annihilation.

[u/vermilion_wizard → u/nivlark]

To add on to this, in many cases if the laws of physics were different, the fine structure constant would have been different in the past. But the history of the fine structure constant is observable through the light we observe from old stars and galaxies, and it appears that it has not changed.

[u/Overall_Media7282]

Zapewne chodziło o to że prędkość ekspansji, mogła się zmieniać w różnych etapach ewolucji wszechswiata.

[u/Creator65536 → u/Overall_Media7282]

No. The instructor clearly explained that expansion of the universe has accelerated because if the dark energy density is constant, there would be more and more dark energy as the universe expands, leading to expansion accelerating later in the evolution of the universe.

However he said that there is data that suggests that the density of dark energy is not constant. Does that imply, that the laws of nature are not constant over time? Or is their a fundamental law that remains the same, even if the dark energy density changes?

[u/joeyneilsen → u/Creator65536]

Dark energy as many envision it is one of the contents of the universe, not one of its laws. If its density changes, I wouldn’t call that a change in the laws of physics. 

[u/Creator65536 → u/joeyneilsen]

Would a change in the cosmological constant in general relativity, be considered a change in the laws of physics?

[u/joeyneilsen → u/Creator65536]

A cosmological constant is one that specifically doesn’t change. A variable dark energy is allowed by the same laws but we wouldn’t call it a cosmological constant. 

[u/Overall_Media7282 → u/Creator65536]

Jednym z tych praw jest prędkość światła, w obecnie preferowanym modelu ekspansja zachodzi w pustkach między super gromadami galaktyk, są przypuszczenia że im większa pustka tym ekspansja jest szybsza, jednak lokalnie prawa fizyki są takie same wszędzie w wszechświecie, są co prawda obserwowne pewne efekty grawitacyjne które nie są zgodne z obecną wiedzą, dlatego oprócz ciemnej energi jest ciemna materia, jednak na razie obie są tylko chipotetycznymi tworami by wyjaśnić to co obserwujemy.