# Tag Info

2

This sort of theory is still an active research topic as can be seen by running this search, which turns up plenty of recent papers related to the topic, including several in credible refereed journals. It is not widely regarded as complete/correct/accepted, but it is grounded in sensible physics... though to me some parts of it are a bit strange.

0

Maybe the Cambridge Cosmology Lecture Notes can help to answer at least the first question. Besides I think you meant annhilation instead of scattering (?), because these equations are for the Dark Matter $\chi$ annihilation process \begin{align} \chi + \bar{\chi} \Leftrightarrow c + d~~~~. \end{align} However, a system is said to be in kinetic equilibrium ...

6

I though I would discuss the transition from radiation to matter dominated phases and from there to the dark energy phase. A fair amount of this can be discussed with just Newtonian mechanics. General relativity changes this by some subtle means, but as a coarse grained view, to borrow a stat mechanics term, Newtonian mechanics captures a lot of this. We ...

16

The title and the text actually ask two different questions. While Kyle Oman and Thriveth answer the title excellently, I'll address the question in the text which asks "Why did the Universe expand in the first place, before dark energy (DE) started to dominate". The answer to this is inflation (we think). The first fraction of a second after the creation ...

9

The short version: The amount of matter in the Universe is fixed, so as the Universe expands, matter density will drop because the same amount of matter will be spread out on more space. Dark Energy, on the other hand, is (by definition) constant or almost constant in density. This means that no matter how dilute the Dark Energy is, if it waits long enough, ...

19

Let's start partway through the expansion of the Universe in the matter dominated epoch. At this time the energy density is dominated by matter, but the dark energy and radiation components are still present, just relatively small. The Universe is expanding, but the expansion is gradually slowing down. As the Universe expands, the density of matter scales ...

-3

Just an idea I'm playing with, There is no dark energy/dark matter. The universe is not expanding into “infinite nothing”, but instead reducing into the amount of space it occupied at inception. Matter is reducing in scale because there is nothing in the universe requiring it to maintain a static size at the subatomic level. The gravitational effect of ...

2

The necessity of GR to solving a particular problem can be assessed by calculating $GM/Rc^2$. Here, $M$ is the mass involved in producing a gravitational field at some separation $R$. The rule of thumb is that if $(GM/Rc^2)\ll 1$ (i.e. is close to zero) then GR effects (time dilation) can be neglected roughly at that order of precision. So if we take an ...

0

The information paradox has no particular connection to electromagnetism. Hawking radiation is not just photons, it's any sort of particle. And Hawking radiation in itself doesn't solve the information paradox - the problem is that Hawking radiation is supposed to be thermal, so quantum information of infalling objects has been irreversibly lost, but that ...

1

Yes. Black Holes (BH) can grow from accreting anything with energy --- including dark matter (DM). I'm not entirely clear on the second part of your question, but probably the most important thing to keep in mind is that the black hole information paradox is still unresolved. Answering how information is not lost for any type of particle, including DM, ...

1

Welcome here. From your profile I see that you are at the beginning stages of learning physics. This is an arduous process that needs a lot of elbow grease in solving problems and/or doing experiments in order to get a basic intuition for the subject. Here is a simplified answer to your questions: Why do most theories about what Dark energy and Dark ...

2

You need to be careful when talking about the extra gravitation created by kinetic energy. It's tempting to think that if you look at a single object then because its relativitic mass is given by: $$m_r = \frac{m}{\sqrt{1 - v^2/c^2}}$$ if you make the object move fast enough it will generate a bigger and bigger gravitational field and even eventually ...

2

The clue is in the name - cold dark matter. A working definition of cold here, is that the total energy is roughly equal to the rest mass energy. Or in other words, that the kinetic energy is very small compared with the rest mass energy.

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