All Questions
352
questions
1
vote
1
answer
71
views
Age of universe vs Hubble time in Milne universe
Consider an empty universe where energy density $\varepsilon = 0$, thus the Friedmann Equation can be reduced into:
$\dot a^2= -\frac{kc^2}{R_O^2}$
$k$ is the curvature of space, $R_0$ is the radius ...
- 411
7
votes
3
answers
5k
views
If we consider the spacetime of the universe to be four-dimensional, does the Big Bang lie in its center?
Apologies for the (hopefully now somewhat less) clickbait-y title. Now, of course, I know that the Big Bang did not happen at any point connected to a single point in our current $3$-dimensional ...
- 1,897
-1
votes
1
answer
53
views
If an area in 2D cannot be curved and finite is the same regarding the space of our pressumed 3D universe?
Is the sentence in the title right that our universe is infinite? And if so does it mean that stars are not evenly distributed along our universe but they all move from a populated centre to a fairly ...
- 2,255
2
votes
1
answer
123
views
Could the universe have a form of a $T^3$-torus?
Cosmological measurements suggest that we live in a flat universe. However, what might be less clear is its topology. So could the flat universe have the form of a $T^3$-torus, i.e. the torus whose ...
- 9,778
1
vote
1
answer
80
views
Cosmic web shape
Does anybody know why the cosmic web is shaped like a web? I feel like it would be more likely that it is more like a galaxy with a supermassive black hole in the center, if that were true, what would ...
0
votes
0
answers
73
views
Are there any ways to conceptualize the relationship between gravity and space-time other than curvatures?
This might sound like a random question, but it came to me while I was trying to conceptualize the size of the universe and started thinking of entire galaxies resembling grands of sand floating ...
1
vote
1
answer
34
views
Question about light and distance
If we see into the past with light and distance travelling so we can’t see things how they are currently, only how they were in the past; and James Webb took a photo from the beginning of the universe ...
- 11
0
votes
0
answers
100
views
Movement of galaxies vs expansion of space
I can readily accept the theory that the universe is expanding as a mathematical model to explain the fact that all galaxies are moving away from each other, but I have difficulty understanding ...
- 1
1
vote
0
answers
60
views
Is the universe closed or flat?
Apparently there is a tension in the measuring of the curvature of the universe (https://arxiv.org/abs/2307.07475) as apparently in 2018 the Planck collaboration got a series of results consistent ...
- 2,462
3
votes
4
answers
3k
views
Is the universe really a closed physical system? [closed]
It is assumed in most physics discussions that the universe is a closed physical system. But is it really? I am defining the universe as the entirety of everything physical, not merely the observable ...
- 1,252
-4
votes
1
answer
78
views
The background of the universe [closed]
Mass(-energy-momentum) curves space(-time).
However - what is space-time?
Is it (the background) necessarily flat(without something in it) ?
If there were an empty universe - is there still spacetime ...
- 63
2
votes
5
answers
1k
views
Why is it that the further a galaxy is, the greater is its recessional velocity?
The exam question is:
Explain how red-shift provides evidence for the Big Bang theory.
One of the points in the answer is:
the further away the galaxy is, the greater is their recessional speed
...
- 21
0
votes
1
answer
46
views
Is it true to say that there is a single electric field and it is inhabited by and affected by all charges in the universe? [closed]
I don't see why or how this could be argued against or if it really matters, but that's why I ask the forum.
I am just trying to understand the nature of the fields in the universe. Am I correct in ...
2
votes
0
answers
58
views
Space expansion or generation [duplicate]
When physicists assert that space is expanding, does this imply the creation of new space? If so, why do they use the term "expansion" instead of "generation"?
- 31
0
votes
1
answer
191
views
Are black holes the edge of our universe?
Are black holes the actual edge of the universe? Because spacetime is another dimension, I would assume the universe doesn’t have perceived corners or edges. At least humans cannot perceive it.
The ...
- 11
1
vote
2
answers
106
views
In spacetime time is a coordinate. Does it mean there is a single objective timeline for the Universe?
If every event can be defined with x, y, z, t coordinates - does it mean all events with the same t are composing the whole Universe at the moment t?
0
votes
0
answers
26
views
What is the expansion of space of one megaparsec in one year?
suppose we know Hubble's constant. In a hypothetical scenario an astronomer finds and confirms Hubble's law for a galaxy 1 mega parsec away.
Next year, how much distance will the hypothetical ...
2
votes
2
answers
414
views
What is the cardinality of intervals in space, and what is the cardinality of intervals in spacetime?
The interval $|(0, 1)| = |\mathbb{R}|$. I naively thought that one could treat intervals in space in kind, i.e., that the cardinality of any interval in space has the cardinality of the continuum. You'...
- 23
1
vote
1
answer
102
views
If spacetime is discrete, would we observe continuous models to show non-rounding and non-truncation errors?
Typically, the ground truth is taken to be the continuous model. Numerical simulations are taken to be the approximation. These simulations deviate from the continuous model due to both a constant ...
- 870
1
vote
1
answer
67
views
Does the universe have an infinite volume? [duplicate]
The implications of a spatially infinite universe is profound, but so are the implications of a finite universe. What we know about this issue?
- 11
0
votes
2
answers
47
views
Is this the Format of the Observable Universe?
The way I have it is: the Observable Universe looks as follows. In some ball, all the galaxy clusters exist, then in a bigger concentric ball the dark ages exist (no galaxies), then on the surface of ...
- 61
0
votes
1
answer
46
views
Average Time since the Big Bang
Can we define an average time for the entire universe relative to the Big Bang and call this the universal time since the beginning of the universe? (time, averaged relative to all possible reference ...
- 613
0
votes
1
answer
122
views
Cosmological principle: can there be a center of the universe "outside" the universe?
I've been watching youtube videos about the cosmological principle. I understand that the expansion of the universe is not concentric (around a specific point in space). The balloon example helped me ...
- 3
-1
votes
2
answers
74
views
The speed of expansion of space in big freeze
In the case of big freeze, space expansion will be accelerating and there appears to be a lot of different phenomena occurring. However, in the case of big rip, the expansion is super-accelerating so ...
- 13
2
votes
1
answer
100
views
Topological phase transitions for the whole universe...?
Physicist Grigory Volovik has put forward some ideas about the universe undergoing a topological phase transition (especially in the early stages of the universe). He published a book called "The ...
- 2,462
0
votes
0
answers
44
views
Effects of anisotropy and non-homogeneity in the universe's symmetries...?
I was reading Philip W Anderson's essay "More is Different" (https://www.tkm.kit.edu/downloads/TKM1_2011_more_is_different_PWA.pdf) and at some point he links the isotropy and homogeneity of ...
- 2,462
0
votes
0
answers
51
views
Was Big Bang the "START" of time? [duplicate]
I know that this question has been repeated a lot. But I still don't understand this concept.
Big bang created matter and space but how could it possibly create time?
If Big bang didn't create time ...
- 77
4
votes
2
answers
950
views
Could it be possible that the Universe is expanding in some areas while contracting in other areas?
I am wondering if could it be possible that the Universe is expanding in some areas while contracting in other areas.
I have wondered if perhaps as one area of the Universe is squeezed inward by some ...
- 478
1
vote
1
answer
81
views
What if the universe was not uniform...?
In this popular science article, they say that if our universe resulted to be non-uniform (that is highly anisotropic and inhomogeneous) then the fundamental laws of physics could change from place to ...
- 2,462
-1
votes
2
answers
83
views
Can a physical object escape the universe? [duplicate]
Can a particle, like an electronic or photon, leave the universe? If the photon for instance travels out toward the edges of the universe, assuming it is flat, will it encounter an invisible wall, or ...
- 13
-3
votes
5
answers
213
views
Can some regions of space (independent of size), be completely devoid of matter?
Before answering the question, keep in mind that I am a second year Biology student, with no experience in studying Physics and a very basic understanding of Mathematics. However, I have some ...
- 69
0
votes
1
answer
105
views
If the universe is infinite, would it be homogenous?
I know, that we can't really know the answer to that, but what is the current state of understanding?
We seem to assume that on very large scales, the universe is homogenous. As I understand it, this ...
- 866
5
votes
3
answers
15k
views
Are the claims about repeating states and space in Netflix's "A Trip to Infinity"'s based on real research?
I just watched Netflix's documentary on infinity "A trip to infinity". They have an example where you put an apple in a perfectly sealed box. They make a claim that seems odd to me.
The ...
- 61
2
votes
1
answer
171
views
Particle horizon in an empty universe
So in this thread,
Can space expand with unlimited speed?,
the author Pulsar made amazing diagrams of different horizons and paths for a benchmark model that describes our current universe, and gave a ...
- 161
4
votes
3
answers
2k
views
Will a light come back within finite years?
In this answer Javier said
Imagine the universe was the inside of a ball. We're 3D now, so no one is hiding any dimensions. This ball has a border, except it's not really a border. You should think ...
- 405
0
votes
2
answers
91
views
What is there at a point the universe hasn't expanded past yet? [duplicate]
(Please don't mark as a duplicate)
If the universe is constantly expanding that means that there is a point the univese hasn't expanded past, with that what would be past that point? This isn't about ...
- 11
0
votes
0
answers
19
views
Dark matter and un-smoothness in spacetime [duplicate]
Since dark matter currently is only observable with its gravitational effects and nothing else can we theorize that dark matter is only non-smoothness in spacetime that has been there from the Big ...
0
votes
1
answer
91
views
If I moved in a straight line forever, would I hit something?
Say you could move infinitely fast/gravity doesn't affect you. You move in a straight line. Do you hit anything, like stars or black holes? Not talking about cosmic dust.
- 178
2
votes
0
answers
85
views
Could the universe be a 4-ball?
I recently thought of the idea that the universe could be an infinite 4-ball. The Big Bang would be its centre, and time would be outward from its centre (one layer would be one point in time). I ...
user334229
2
votes
2
answers
178
views
Space-time continuum expansion
I still don't understand how the expansion of the universe works.
If the universe is made up of an infinite number of points that make up space-time, then how can space expand or stretch. Common sense ...
- 343
-1
votes
2
answers
169
views
Can space only be infinite? [closed]
I have read before that if you could just go fast enough, as a thought experiment, and you move in a straight line, in any direction, that you eventually might reach the spot from which you started. I ...
- 11
1
vote
3
answers
136
views
Is the solar system sitting in the centre of curved spacetime and if so, are we viewing the rest of the universe from inside that "bubble"?
I read an article about a huge bubble being discovered in which the solar system sits bang in the middle. It got me thinking about the curvature of spacetime. The bubble was created by several ...
3
votes
2
answers
1k
views
Space is flat but spacetime is curved?
In the picture below a triangle has been drawn on a spherical shaped object. The angles add up to 67+48+73=188 degrees. Since the surface of earth is also almost spherical therefore an experiment can ...
- 717
-2
votes
1
answer
261
views
IF heat was the absence of cold, how would that change physics [closed]
Back in high school, I had an argument with my physical science teacher. She said cold is the absence of heat. I asked why it couldn't be the other way around. She simply stated that was not possible. ...
0
votes
3
answers
242
views
Is there evidence for the existence of time? And what's eternalism (block universe)? [closed]
I have been told that: "time is an illusion. Rather than a chronological progression of events, all events occur simultaneously in space, and so right now, dinosaurs exist, WW2 just ended, you're ...
- 133
-5
votes
2
answers
399
views
Is time continuous or discontinuous? [duplicate]
As I notice I sometimes feel as if time is discontinuous it’s like a comic book where each act is planned out and the main character just comes there. Please give an explanation and correct me if I am ...
- 21
0
votes
0
answers
44
views
Question: Is our Universe finite? [duplicate]
Is it at all possible to compare our three-dimensional space with the two-dimensional surface of an expanding ball where recombination-induced radiation from the point at the top of the ball travels ...
- 2,255
0
votes
1
answer
100
views
Is space build of smaller space-particles?
In atomic physics one of the most decisive proves that the objects are build of atoms is that they can be deformed and twisted. So is it not logical to believe that space itself is also build of ...
- 651
0
votes
3
answers
793
views
Why are things far away moving away faster?
This is a really dumb question. But if things farther away are red shifted more, that literally means that the further away in time, things were moving away faster, but things closer to us arent ...
- 59
-1
votes
1
answer
1k
views
Did gravity exist before the Big Bang?
Gravity is one of the biggest mysteries. It is the weakest of the 4 fundamental forces but we have no clue how it works. Einstein never was able to figure it out in his lifetime. There is evidence ...
- 129