Questions tagged [universe]
The universe refers to the cosmos; all of space-time and that which exists as part of it. Alternatively, it can refer to the observable universe, which only contains the part we can see. Questions tagged with this should ask about physics at scales the size of the universe or specific properties of the universe
1,753
questions
1
vote
2
answers
736
views
Why do moving liquid metals generate a magnetic field?
The earth's magnetic field is generated due to the movement of liquid metals, but why not solid? Why is it that moving molten metals create a magnetic field, but when nothing of the sort happens when ...
- 257
0
votes
2
answers
130
views
How do we know if a particle is moving or not? [duplicate]
if there is only one particle in the universe, how do we know if it is moving or not?
We don't have any other object to track the distance between the two, then is it possible to determine the ...
- 103
3
votes
2
answers
144
views
Does spacetime bending contradict the universe following euclidean geometry?
According to experiments the universe is believed to be flat, meaning that it would follow euclidean geometry. However, is that compatible with the fact that spacetime bends due to gravity? Does ...
- 31
0
votes
0
answers
78
views
Suppose you are told that the linear size of everything in the universe has been doubled overnight. Can you test this statement? [duplicate]
Suppose you are told that the linear size of everything in the universe has been doubled overnight. Can you test this statement by measuring sizes with a meter stick? Can you test it by the fact that ...
1
vote
2
answers
192
views
Are galaxies and clusters of galaxies much denser when we observe the 'past' far universe than the 'present' closer part of the universe?
Are galaxies and clusters of galaxies much denser when we observe the 'past' far universe?If the universe is expanding, the galaxies should have been much closer in the past so should we see a larger ...
- 1,825
18
votes
2
answers
4k
views
How long ago was the Universe small enough for interstellar travel?
Currently, even the nearest stars are lightyears away, and impossible to reach in our lifetimes. If space is always expanding, and was once infinitely smaller, then at what point in the past was space ...
- 709
0
votes
1
answer
106
views
How could the concept of finite unbounded universe be compatible with the fact that there are 3 dimensions? Is the example of the balloon a bad one?
In cosmology, the current understanding is that the universe is finite while being unbounded.
To explain that, one often uses the example of the surface of a balloon: if we are on the surface of the ...
2
votes
2
answers
121
views
Is it right to assume that most physical values always drop to zero at spacial infinity?
In many parts of physics, some physical quantities are supposedly considered to go to zero at infinity. For example, in classical field theory, we often use Gauss's Law to turn volume integrals into ...
- 94
2
votes
1
answer
150
views
Can the universe be considered an isolated system?
Is it allowed to consider the physical universe as a thermodynamic isolated system obeying the general second law since the total entropy in a comoving volume does not decrease with time despite ...
- 830
9
votes
3
answers
2k
views
Is "speed relative to the universe" a well-defined concept? [duplicate]
Prompted by commenting on this question.
I offered the standard "Which frame of reference are you using? Yours? A satellite's? The sun's? The Milky Way's?" observation.
Which prompted me to ...
- 659
0
votes
1
answer
128
views
Shape of the universe as dark energy starts to dominate
We say that the universe has a particular shape (flat, sphere or saddle) depending on what the ratio between the average energy density and the critical energy density is, something we call $\Omega$.
...
1
vote
3
answers
388
views
Why the mechanism of everything in the universe has a pattern? [closed]
why everything in the universe has a pattern which can be identified and understood to determine outcomes, properties, effects of almost everything. I am saying that couldn't the universe be like ...
- 51
1
vote
3
answers
1k
views
Does quantum physics apply to other dimensions?
Does quantum physics apply to other dimensions? Common sense seems to dictate that in a universe with two dimensions, quantum physics wouldn't apply, because particles would be vastly different from ...
- 791
3
votes
2
answers
265
views
How can spacetime be expanding faster than the speed of light? [duplicate]
How can spacetime be expanding faster than the speed of light when the speed of light is the speed limit of the universe?
- 117
21
votes
2
answers
5k
views
Did we really determine a positive curvature of the Universe in 2019?
This arXiv paper says:
The recent Planck Legacy 2018 release has confirmed the presence of an enhanced lensing amplitude in CMB power spectra compared to that predicted in the standard $\lambda$CDM ...
- 8,221
33
votes
9
answers
8k
views
Is space really expanding?
In a book called "Einstein, Relativity and Absolute Simultaneity" there was this sentence by Smith:
There is no observational evidence for a space expansion hypothesis. What is observed are ...
user311398
1
vote
0
answers
34
views
How the Big Bang/Inflation started? [duplicate]
Big Bang was the start of everything. At first, there was energy just confined to a small space. And then it expanded rapidly, which we call Inflation. But what caused this Inflation, and how that ...
- 133
8
votes
4
answers
3k
views
Confusion about the shape of the universe
From what I understood, a Universe with a positive curvature should behave like a sphere, and light will eventually reach the point where it started. Similarly to a ball, when an ant starts to walk on ...
user311398
0
votes
4
answers
185
views
Is it possible that the center of the universe is outside our observable universe?
Is it possible that the universe does have a center after all, but we just cannot see it because it already fell beyond the event horizon of our observable universe?
If not, how do we know this for ...
- 103
0
votes
1
answer
114
views
Time and causality at the beginning of the universe
I have been, quite a few times, been caught up in arguments on the internet, where my opponent posits causal events existed before the "singularity" at the "beginning" of the ...
- 1
3
votes
3
answers
322
views
May the space be flat and infinite or curved and finite?
May the space be flat and infinite or curved and finite? Personally I cannot explain myself a infinite object and how eventually to describe it but on the other hand a curved and finite space should ...
- 2,219
0
votes
0
answers
29
views
Quantum tunneling wave function derivation
In Vilenkin's paper Quantum cosmology and the initial state of the Universe, We find the tunneling wave function to be $ \psi _{T}=\frac{Ai(-z)+iBi(-z)}{Ai(-z_{0})+iBi(-z_{0})}$ (4.27 in the original ...
- 11
3
votes
1
answer
257
views
How could the universe be spatially flat on average, if all forms of energy have positive spatial curvature?
EDIT: I stated the current understanding wrong, thanks Koschi for the comment.
In the $\Lambda$CDM model, it's said that the vacuum energy pretty much "balances" the baryonic matter, such ...
- 2,423
1
vote
2
answers
252
views
Parameterizing a universe with non-zero curvature, some matter, no dark matter, and no dark energy
Defining $\Omega_i$ by $\rho_i (t_0) = \Omega_i \rho_{c_0}$, we can obtain the below equality.
$$H^2 = H_0^2 \left(\frac{\Omega_r}{a^4} + \frac{\Omega_m}{a^3}+\frac{\Omega_k}{a^2} + \Omega_\Lambda\...
0
votes
0
answers
34
views
What is the dark energy & dark matter? [duplicate]
Can anyone explain to me simply what is the dark energy or what is the dark matter. I have been trying to understand it deeply. but I somehow failed.
0
votes
0
answers
24
views
What was the volume of the universe a short time after the big bang? [duplicate]
I assume this question is somehow ill posed, but I do not know in which way.
I think it is not a difference here whether the universe expanded into space,
or space expanded itself.
Seen from the ...
- 3,891
0
votes
3
answers
700
views
Why do they say universe will become cold with expansion?
The Universe is expanding and they say it will eventually become cold, and new star formation will stop.
But galaxies are only moving away from each other, and each galaxy itself stays intact and the ...
- 283
1
vote
0
answers
187
views
Energy density of particle species in thermal equilibrium
I am reading the book Kolb and Turner "The Early Universe". In the thermodynamics section they mention that the total energy density of different species in equilibrium is
$$\rho=T^4\sum_{\...
user255856
1
vote
0
answers
43
views
If atom ionization energy increases, how will the Cosmic Microwave Background (CMB) radiation temperature change today?
I know that the Cosmic Microwave Background (CMB) radiation is the leftover radiation of the photon emitted by the last cosmic-scale combination of electrons and ions to neutral atoms in the history ...
- 11.8k
2
votes
0
answers
104
views
Why is life impossible in the supersymmetric Minkowski vacuum?
I read in several sources that the existence of life is impossible in the supersymmetric Minkowski vacuum (also called the terminal vacuum, the true vacuum or the global minimum). Why? After all, ...
- 930
0
votes
0
answers
44
views
If the universe is 13,7 billion years old how it could be infinitely large? [duplicate]
If the universe is 13,7 billion years old how it could be infinitely large? Maybe it is curved to account for being finite? But then there should be more spatial dimensions...is it right?
- 1,825
1
vote
1
answer
978
views
What is time till false vacuum decay? (Which source is in error?)
https://arxiv.org/abs/1308.4686 says that the time till false vacuum collapse is just about the age of the universe:
ΛCDM ... can be achieved if the top quark pole mass is approximately 178 GeV
That ...
- 375
2
votes
4
answers
59
views
Could an external being predict quantum uncertainty given all the information of our Universe? [duplicate]
I'll elaborate and be more specific. I understand that this is almost a metaphysical question but nonetheless I want to give it a try.
Could an external being outside our Universe create 10 ...
- 401
0
votes
1
answer
201
views
How is energy in the universe constant? [duplicate]
We have been taught that the universe has a constant amount of energy but if energy can be converted into matter, how is it constant?
- 87
-1
votes
2
answers
130
views
Block universe, a silly question [closed]
In a block universe we can describe an object using a function: F(x,y,z,t)
Now, let’s take a brand new empty universe and add to it an object:
F(x,y,z,t)
We know everything about that object, so, no ...
- 99
2
votes
1
answer
105
views
How can we be sure that gravity is not the reason for the perceived expansion of the universe? [closed]
The question has been partially discussed here but I believe the following formulation is a bit different.
What experience could disprove the notion that the universe is not expanding despite the pull ...
- 145
-1
votes
1
answer
76
views
Why should there be a Big Crunch?
Everyone knows it all started with the Big Bang. And then on, all objects have been moving away from each other very fast. And this rate itself is accelerated, according to Hubble's Theory.
However, ...
- 1,630
0
votes
0
answers
41
views
In general relativity, could our curved spacetime be embedded in a higher dimension flat spacetime (5D+1) of the compatible metric? [duplicate]
I took General Relativity at university, years ago and have a question that has recently occurred to me. This might be a dumb one, so I apologise if the answer is a well known negative.
Mathematically,...
- 1,087
0
votes
1
answer
106
views
How does space change as the universe expands?
Consider a metre ruler. Despite the universe – and space itself – constantly expanding, the ruler maintains its size. If this ruler was alone in empty-ish space, other distant objects would appear to ...
2
votes
0
answers
68
views
What would happen if universe wouldn’t accelerate faster than speed of light? [closed]
Its quite stunning to assume that not only space-time is expanding but also the rate of expansion is greather that speed of light.
But after the initial surprise, I ve been wondering if that fenomena ...
- 21
0
votes
1
answer
107
views
Are there absolutely isolated quantum systems in the Universe?
The university began to study quantum entanglement, and the teacher said that in order to get entangled, systems must have low entropy. I did not understand this point very well, that is, systems with ...
- 930
2
votes
3
answers
112
views
Is my intuitive understanding of physical infinity correct?
Quite possibly I got the section wrong again but the question is about cosmology. When they talk about an infinite Universe, they do not mean that in such a Universe there are two objects, the ...
- 930
2
votes
3
answers
1k
views
How can the universe possess rotational symmetry yet have no center?
From Noether's theorem we understand that conservation of angular momentum means that the laws of nature have rotational symmetry. From cosmology we understand that the universe has no center. But ...
- 139
3
votes
2
answers
983
views
Is concept expressed in "Autodidactic Universe" article plausible? [closed]
As I understood, the authors (Lee Smolin et al) of the "Autodidactic Universe" article suggest that the fundamental laws of nature as time progresses since the Big Bang event (which happened ...
- 386
0
votes
0
answers
90
views
In what sense can the Universe be thought of as a true isolated system?
I've seen the question asked before, but I didn't find the way it was asked or the answers to be satisfactory. I must admit I am no expert, so I hope for answers suited more for the layman, but as ...
- 1
3
votes
1
answer
82
views
How do galaxies cross our particle horizon?
At the begin of his lecture "The Quantum Origin of the Universe" (2014) James Hartle makes the following statement:
"A new galaxy with a 100 billion stars becomes visible - ie comes ...
- 830
1
vote
1
answer
75
views
Why does the character of the universe not change as it evolves?
This question assumes Newtonian cosmology which considers a pressureless dust model of the universe. According to Carroll & Ostlie's Introduction to Modern Astrophysics textbook, I quote directly:
...
- 183
0
votes
1
answer
192
views
Are unreachable parts of the universe 100% unreachable or do we just say so?
In Kurzgesagt's video TRUE limits of Humanity is it stated that most of the universe will be forever unreachable because the universe is expanding faster than the speed of light. This is possible ...
- 123
2
votes
0
answers
41
views
What mechanisms could keep Universe at 0 energy?
In my understanding is right, there is no obvious guarantee of the conservation of energy on the cosmological scale. The expansion of space-time causes the red-shift of radiation and therefore the ...
- 180
0
votes
0
answers
38
views
Evolution of the temperature of electron proton gas in universe after recombination
How can I mathematically find the evolution of temperature of matter in universe over time?
Suppose I start with a mixture of free electrons and protons. After sometime, they will cool down, and a ...
- 864