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

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55 views

Explain why the universe could be compact

Regarding the topology of the universe, it could be compact like a sphere or open like a Euclidean space, but since the universe started from a single point, doesn't that mean that the shape of the ...
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On the assumption that the universe is not expanding, but things are shrinking [duplicate]

Although the title gets the gist across, clearly, this is going to run into a lot of contradictions if I don't explain the idea as it resides in my head. Bear with me in the following analogy of a ...
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Proof of an accelerated expansion of the universe

Are there any more "proofs" of an accelerated expansion of the universe other than: $1:$ that galaxies that are farther away from us are also more redshifted. $2:$ using "Baryon ...
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3answers
92 views

Is a universe with zero Planck's constant possible? If so, what is the behaviour of light and matter? [closed]

A universe in which Planck's constant is zero has no quantum physics i.e, is not governed by quantum physics. If we assume such a universe exist, will radiation especially light still exist? Will ...
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Why do the TT, TE and EE power spectra in the CMB lead to different values for the Hubble constant?

Table 2 of Planck 2018 results Why are these three values different? In particular, why is the EE value so much closer to the local universe value, and have so much higher error?
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Can we really apply the second law to the entire universe?

I do not doubt the second law in general, just if it rigorously applied to the entire universe. Here's why I ask this 2nd law - restricted to isolated systems: "The second law may be formulated ...
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From which dark matter made of? [closed]

Observations indicate that about 90 percent of the matter itself in the universe is made up of dark matter. Exactly what kind of things dark matter made of?
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What do we meant by “Thermally isolated universe”?

As we knew in general, $$dS=\frac{\delta Q_{rev}}{T}\geq \frac{\delta Q}{T}.$$ Consider a thermally isolated system so that the above inequality becomes $$dS\geq 0 \ \ \text{if} \ \ \ \delta Q=0. $$ ...
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What if dark matter occupied three other dimensions and it could only interact with our three dimensions through gravity and time? [closed]

I recently thought about dark matter existing in three other dimensions similar to ours but wasn't sure if it was logical. It would be of great help if someone would give me a clear insight into this ...
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3answers
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Directional dependence of scale factor $a(t)$

Why does the scale factor $a(t)$ in FLRW metric has only temporal dependence but not directional? If $a(t)$ had a directional dependence, I think it would violate the cosmological principle since The ...
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2answers
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Is the commonly accepted Universe age of 13.8B years really accurate? [closed]

Hmm...challenging the error margin of a specific calculation result of a specific model based on confirmed inconsistencies is regarded as "opinion-based". Interesting. Just curious, is the ...
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1answer
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Amount of matter in two infinite universes [closed]

I don't know exactly where to find any other information on this question so I thought I would ask here. If there were two infinitely large universes one where 20% of the space in the universe was ...
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While space itself expands, why does it seem to expand more between the stars than between me and my desk? [duplicate]

this is my first post in this community and I am not entirely sure if it is appropriate. I am not an educated physicist, just an ethusiastic person trying to understand the universe. So I apologise if ...
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Particle fields and field theory

All particles have their own fields. More precisely, I've learned that an electron is just a bundle-up packet of energy from an 'electron field'. So as in this sense, all particles (from the standard ...
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Can laws of physics suddenly change? [closed]

At the present time, we have a collection of variables and laws that describe, maybe in a non deterministic fashion, the evolution of the universe. Is it possible that these laws suddenly change? Say, ...
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Why the FLRW metric was chosen with constant $g_{00}$ time component?

The FLRW metric is used to describe Universe expansion. Why for this purpose time component of the metric is chosen to be constant? Even other metrics which describe inhomogeneous Universe expansion ...
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Excess number density of fermions in the Universe

In Kolb and Turner's book, it is given that if the chemical potential of the fermionic species if not taken to be neglible then the excess fermion number density is given as follows. $n_+$ - $n_-$ = $\...
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1answer
71 views

Why does hyperbolic universe must be infinite?

I remember back in the day, I took a course in cosmology where I was taught that there are three possible types of universes based on their curvature, the flat universe which may be compact or ...
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Euler equation in comoving coordinate

I have the following cosmology exercise : I found many hints on internet : first source, second source But I still can't figure it out. If we is start from \begin{equation} \frac{\partial \vec{v}}{\...
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If the universe is expanding at an accelerated rate, was it stationary at some point in time and contracting before that?

If the universe is expanding at an accelerated rate it would mean that the more we go in the past, the slower the speed of expansion must have been. Provided the acceleration was always non-zero and ...
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Where was matter before the big bang? [duplicate]

I read that big bang caused all the matter to spread in the universe and it happened by a gigantic bang from a tiny ball of matter. But where was matter before it? From where did all the stuff came ...
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1answer
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How does universe expand when cosmological constant is zero?

From what I learned, Einstein believed in a static universe but from his general relativity equations universe must collapse under gravity. Hence Einstein adjusted this gravity with cosmological ...
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Why energy cannot be created nor destroyed if the Universe is still growing in size? [duplicate]

Why energy cannot be created nor destroyed if the Universe is still growing in size?There is creation of new space and infact this space is so energetic that causes 'Hubble flow' of matter...
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Could the laws of physics have existed before the universe? [closed]

Alexander Vilenkin writes that perhaps the laws of physics could have existed even before the universe emerged from nothing. How could the laws of physics exist without the existence of the universe ...
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Is everything that has happened, is happening and will happen just a reaction to the action of Big Bang? [closed]

I think the question is vague but interesting. I was wondering if we trace backwards the cause of any event, and then cause of that cause and so on... where will we end up? Is there just one trigger ...
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Does such a variant of the large-scale structure of the Universe make sense?

Although the observed universe is considered homogeneous, there are small local inhomogeneities in it. These are galaxies with higher density, and voids with lower density. In the region of galaxies, ...
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Seeking a plot of the energy history of the Universe

How can we plot the energy density $\rho$ as a function of the age $t$ of the universe (or temperature $T$) from the time of radiation domination to the time of cosmological constant domination with ...
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How can we talk about a single temperature if the early universe was in local thermal equilibrium?

If the early universe was only in local thermal equilibrium, how can we talk about a single temperature for the whole universe?
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Is the present pressure in the universe negative?

I read from cosmological books that the pressure associated with the dark energy component, which is responsible for the accelerating expansion of the universe, is negative. Is the total pressure of ...
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Homogenous Universe (Earthlike planets?)

The fact that the Universe is homogenous can lead to a conclusion that there are another earthlike planets, or planets like earth are considered as some differences that are allowed in the definition ...
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1answer
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Universe with another kind of particles [closed]

Could we imagine a universe with particles having types and properties different from those which exist in our universe?
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What known laws of physics give support on the Big Bang Theory? [closed]

What known laws of physics give evidences that supports the Big Bang Theory? Gravity is common, but what other laws of physics can support the theory?
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How to define a temperature for the early Universe?

The concept of a temperature $T$ is routinely used in discussing the thermodynamics of the early Universe. It enters the discussion via equilibrium phase space distribution of various particles that ...
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4answers
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Do the laws of physics describe the properties of the universe? [closed]

What do the laws of physics describe? Do they describe the properties of the universe? Or does the properties of the universe described by physical theories, and laws describe how those theories work?
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How did the Universe behave just at the end of inflation?

Edit When the inflaton field $\phi$ rolls down the potential $V(\phi)$ to the value $\phi=\phi_f$ where the slow-roll parameter $\epsilon(\phi_f)\to 1$, inflation ends. By calculating the behaviour of ...
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Does the Cosmic Background Radiation (CMB) indicate that we may be any kind of privileged observer?

To first order, the Cosmic Background Radiation (CMB) looks the same in every direction. Apparently, however, at the level of parts per million, there are small distinctive variations in the ...
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Why does gravitational attraction result in a decelerating universe

I read the following statement: There is matter and energy in the universe, and their mutual gravitational attraction will slow down the expansion of the universe, leading to a monotonically ...
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1answer
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How much of the universe will get flatter in how long?

So I learned recently that star systems are relatively flat due to the conservation of angular momentum. On how large a scale does this degree of flattening apply. i.e will the universe eventually ...
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How can the universe be a computation?

A few physicists (or computer scientists) take it for granted that the universe is a computation. However, I am not able to understand how the universe CAN be a computation in the first place. I come ...
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Is energy of universe constant? [closed]

First of all please do not flag this question as duplicate(I have read all the others like this one, this one and this one is just talking about observable universe. none of them talked in this ...
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Is the temperature of the universe rising?

I know the temperature of the universe is decreasing due to it's expansion after the big bang but after I came up with this article in AOP(please note I don't have the access of the journal,so I have ...
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Are there theories about the end of the universe where time ends?

We always tend to think that future will come no matter how far in the future that is. But according to some theories, time started at the big bang and the term "before the big bang" doesn't ...
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What factors affect what happens in the far, far future of the universe, after we undergo a heat death?

I've been reading about Boltzmann brains, especially Sean Carroll's stuff, and am interested in the scenario he describes in which, after the heat death, the universe is populated with objects ...
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How do scientists know the min limit of temperature is -273 degree celsius?

How do scientists know the min limit of temperature is -273 degree celsius? I wonder how do scientists confirmed that there is no place belong to less than -273 degree celsius in the universe? Why the ...
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Why is the universe so stable and consistent? [closed]

Why is it we can discover patterns? Like math, chemistry, physics, biology, etc. Why does the universe never fail us? For example in one second I'm standing next to you, the next I'm still standing ...
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Is Loop quantum cosmology replacing the Big Bang singularity with a big bounce?

Am I correct in understanding that in the cosmological version of Loop quantum gravity, namely Loop quantum cosmology, the universe in all models starts with a big bounce? Are there other models, for ...
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Could the world be represented as binary? [duplicate]

The Matrix the movie regards the whole world as being binary and us living in a computer simulation. Could the world be represented as binary similar to a matrix or is that not practical give quantum ...
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111 views

Can the universe be considered a thermodynamic system?

With matter a type of energy, can/should the universe be considered a thermodynamic system? At what scale would it be relevant?
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What is the equation that tells us that the universe is expanding?

Well I know that the universe is expanding, but is there any equation or letter or simply just an expression that tells us that the universe is expanding...

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