All Questions
45
questions
194
votes
15
answers
31k
views
Why does space expansion not expand matter?
I have looked at other questions on this site (e.g. "why does space expansion affect matter") but can't find the answer I am looking for.
So here is my question: One often hears talk of space ...
- 2,055
50
votes
8
answers
7k
views
Why isn't dark matter just ordinary matter?
There's more gravitational force in our galaxy (and others) than can be explained by counting stars made of ordinary matter. So why not lots of dark planetary systems (i.e., without stars) made of ...
- 603
23
votes
1
answer
9k
views
How do we know Dark Matter is non-baryonic? [duplicate]
It seems widely stated, but not thoroughly explained, that Dark Matter is not normal matter as we understand it. Wikipedia states "Consistency with other observations indicates that the vast majority ...
- 3,241
12
votes
2
answers
2k
views
What is the physics definition of space?
When astronomers, physicists say that space is expanding, does the definition of space include matter, energy and the particles within?
Or is space an empty 'stage' in which all of the above exist?
...
- 11.7k
6
votes
4
answers
598
views
Which one is more fundamental in nature: matter or radiation?
I am following a geometric perspective on abelian gauge theory as done in the lecture notes by Timo Weigand, chapter 6, pp 165-167, here:
http://www.thphys.uni-heidelberg.de/~weigand/QFT1-13-14/...
user56963
6
votes
1
answer
354
views
How would the universe look like without matter?
I was wondering how the universe would look like if it would have been perfectly symmetrical in terms of matter and antimatter. If I understand correctly, there would be no "particle" but ...
- 357
5
votes
3
answers
2k
views
Creation of matter in the Big Bang
I appreciate your patience to my neophyte question. I am working on my dissertation in philosophy (which has nothing or little to do with physics) about the "problem of naming." Briefly what ...
- 59
5
votes
3
answers
237
views
Cosmological Inflation: If Photon expands, why not other matter?
I keep repeatedly reading in many Stack Exchange and Quora questions that space of universe expands but particles (matter) don't, see e.g. this Phys.SE post. The reason given is that particles are ...
- 149
4
votes
2
answers
492
views
Intuition for why matter dominated expansion is faster than radiation domination?
In a matter dominated universe $a_{\rm mat.}(t)\sim t^{2/3}$, while in a radiation dominated universe, $a(t)_{\rm rad.}\sim t^{1/2}$. Therefore, a matter dominated universe is expanding more quickly, ...
- 3,492
3
votes
3
answers
204
views
Expansion of space or contracting of matter? [duplicate]
Can the expansion of the Universe effect be explained by contraction of the overall matter instead? That is, all material objects are in fact contracting, due to what the universal space seems (and ...
- 31
3
votes
3
answers
2k
views
Does our universe have an even distribution of matter in every direction
It occurred to me that it does since gravity in the space I exist appears to be uniform Ie there is no net movement of objects I observe in a particular direction. I maybe speaking from classical ...
- 101
3
votes
1
answer
567
views
What is the difference between matter & spacetime? [duplicate]
If the universe is expanding why doesn't the matter in it expand proportionally making it seem as if the universe is static? Alternatively, as spacetime expands why does it not just slide past matter ...
2
votes
1
answer
93
views
Total number of particles in the universe (according to Planck Surveyor measures, 2018)
Recently, Marco Ajello et al. (2018) has estimated the total number of photons in the whole observable universe as:
$$N_\gamma \approx 4\cdot 10^{84}.$$
On the other hand, the ratio of baryons to ...
- 1,640
2
votes
1
answer
488
views
Is the expansion of space universal or local?; does matter impede the expansion of space?
Michael Strauss in his recent book "Welcome to the Universe" with authors Tyson & Gott describes that space is NOT expanding within galaxies but rather between galaxies. So then the expansion is ...
- 11.7k
2
votes
2
answers
464
views
Why do matter and radiation densities scale differently in expanding universe, when all matter is energy and all energy is matter? [duplicate]
I sometimes see the first Friedmann equation expressed with different terms for the density of "matter" and the density of "radiation". It is said that with an increase in scale ...
- 213