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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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?
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Is there a physics model where we can produce infinite work? [closed]

Looking at this question Is the universe a perpetual motion machine? you might ask yourself does the universe(with stars and planets) have an end? Many physicists believe the planet ends with heat ...
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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 ...
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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 ...
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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?
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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 ...
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94 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 ...
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54 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, ...
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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,...
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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 ...
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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 ...
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Adiabatic and Entropy Perturbations in Cosmology

This is a part from this paper (https://ui.adsabs.harvard.edu/abs/1992PhR...215..203M/abstract) by Mukhanov on Theory of Cosmological Perturbations. The cosmological perturbations are broadly ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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Specification of nature of light

Light travels in both wave and particle form then how we able to know when it will travel in wave and when in particle form.
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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 ...
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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: ...
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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 ...
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Why does Tegmark think physical objects are unique? [closed]

I'm reading Max Tegmark's (MIT) article The Mathematical Universe. According to Tegmark's hypothesis, the Universe is not simply described using mathematics, but is itself mathematical, that is, any ...
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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 ...
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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 ...
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Kinematic behavior of a flat, a closed, an open universe

according to An Introduction to Modern Astrophysics, 1263p, there is explanation about evolution of scale factor. “ For the early universe($R<1$) there is little difference among the kinematic ...
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Are the constituents of the known universe matter (including antimatter) and electromagnetic radiation?

Is it correct to say that the constituents of the known universe are matter (including antimatter) and electromagnetic radiation, leaving aside the comparatively hypothetical ‘dark’ entities? This is ...
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Why does starting point of graph differ?

Why open universe, flat universe, closed universe have a different starting point? I know they have different age. I want the solution using mathematical method
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The end of the Universe [closed]

I was reading about antimatter and dark energy and how Hawking Radiation can destroy black holes when I came across three theories about how the universe might end. There are various theories ...
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Where does Universe occupy? [closed]

As we know if there is something that must occupy some spaces(Maybe, some Quantum "things" don't occupy spaces). I was thinking of outer-universe. As we know every object/things occupy ...
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Einstein's Static Solution with $\Lambda =0$ to The Friedmann Equations?

Is it possible to show the universe is static (i.e., $a=a_*=\rm{const}$) without assuming $a=\rm{const}$ to begin with, and using a mix of $w=0$ and $w=-1$? Let $\Lambda=0$ in the Friedmann equations, ...
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Static solution to the Friedmann equation [duplicate]

I want to find the static solution for the Friedmann equation: $$ \bigg(\frac{\dot{a}}{a}\bigg)^2=H_0^2\bigg(\Omega_m\bigg(\frac{a_0}{a}\bigg)^3+\Omega_v+\Omega_k\bigg(\frac{a_0}{a}\bigg)^2\bigg) $$ ...
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Cosmology - Confusion About Visualising the Universe as the Surface of a 3-Sphere

Consider the FRW metric for the Universe in the form found in many standard cosmology textbooks: $$ds^2 = -dt^2 + a(t)^2\left(\frac{dr^2}{1-Kr^2}+r^2(d\theta^2 + \sin^2\theta d\phi^2)\right)$$ I am ...
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Gauge invariance prohibits the existence of other universes?

I'm only a student, so most likely misunderstood. The Wikipedia article on Relational quantum mechanics says: "The universe is the sum total of everything in existence with any possibility of ...
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Problems related to conceptual understanding of the hypothesis of the mathematical universe [closed]

The teacher gave a long lecture about Max Tegmark's hypothesis of the mathematical universe, and I realized that apart from Tegmark himself and his followers, few people understand the concept of the ...
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Can the “Boltzmann brain” concept inform discussions of cosmology?

The Boltzmann brain was originally discussed as a sort of thought-experiment or aid to reflection on what might possibly happen in the universe. Its first discussion was in the context of thermal ...
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Why is this statement “The number of negative and positive charges in the Universe does not change” false?

I am not able to understand why this is false given that total charge in a system is conserved. Does this mean that charge can be created or destroyed such that sum of charge remains same?
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If the universe is infinite, then how can we tell it's expanding? [duplicate]

I understand that our universe is finite to a point that we can see it expand but if that's the case, how is it also frequently called 'infinite'. While there are similar questions out there I would ...
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Can a cosmological constant model inhomogeneities?

Consider the following zero-order approximation to the universe: Spacetime is perfectly homogeneous, and The cosmological constant is exactly zero. This doesn't quite work. Neither assumption is ...
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Is limited computational capacity a fundamental obstacle?

Statistical physics books often motivate the necessity of statistical/thermodynamic description by impossibility of calculating the trajectories of all the molecules (I speak of "trajectories&...
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Entropy of the Universe and Quantum Information [closed]

From what I have understand, heat death of the universe requires a positive cosmological constant. And so when the universe achieves thermodynamic equilibrium, it shall have maximum entropy. Now I ...
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Will a neutrino gas always stay at the same temperature?

Does the neutrino gas pervading the universe always have the same temperature? Does it still have the temperature as the neutrino gas that emerged from the big bang? Neutrinos don't interact, so how ...
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106 views

Conservation of Charge

We know that universe is all about charge conservation mass conservation and energy conservation. Charge on a system can be measured by comparing it with charge on a standard body, positive and ...
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Could the observable universe be bigger than the universe?

First of all, I'm a layman to cosmology. So please excuse the possibly oversimplified picture I have in mind. I was wondering how we could know that the observable universe is only a fraction of the ...
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Will time exist in heat of the universe? [duplicate]

Will there be any sense of time during the heat death of universe? Since there is no interesting events I think you cannot measure time.
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What does the concept of an “infinite universe” actually mean?

When physicists talk about the universe being infinite, or wondering whether it is or not, what do these two options actually mean? I am not interested whether the universe is infinite or not, I am ...
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Geometric features of a closed finite universe

I am a student, so the question may sound silly. If the 2-sphere is the surface of a ball, that is, it is embedded in a three-dimensional space, then the 3-sphere must also be the surface of a four-...
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Temperature of Closed Universe in future

This is a question from "Introduction to Modern Cosmology" by Andrew Liddle: Suppose we live in a closed universe(k > 0), which will recollapse some time in the future. What will the ...
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If we were able to prove that the Universe is infinite, wouldn't that statistically prove that there is no other forms of life?

I want to begin my explanation using abstract mathematical explanation to repetition possibility by taking independent samples $X_n$ from some continuous probability distribution: https://math....
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141 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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