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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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3answers
121 views

How much photon energy has already been destroyed?

We get taught in school that energy can neither be created, nor destroyed. The law of energy conservation is confirmed by many processes. It is an incredibly accurate assumption for every-day life. ...
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1answer
37 views

Doesn't universe violates law of conservation of linear momentum?

As mass of universe is not constant,it means that its linear momentum is not constant.But thinking in another way,is universe subjected to a net external force?But what is external for universe?...
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20 views

How has the size of the universe changed as a function of time? [duplicate]

Is there a plot or an equation that calculates/shows the size of the universe for each era? Preferably starting from the Big Bang, inflation and on up to today.
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1answer
55 views

Positive local spatial curvature of the universe implies that the universe is compact (i.e. finite)?

I quote from the Wikipedia page about the shape of the universe: If the spatial geometry [of the universe] is spherical, i.e., possess positive curvature, the topology is compact. I'm trying to ...
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1answer
62 views

What happened to a photon in the early universe?

So in the early universe where the universe was really small what would happen to a photon is it didn't collide with any material and made it to the edge of the universe? Because yes the big bang ...
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0answers
34 views

In cosmology, what means the 'Fourier modes'?

I am reading papers of cosmology for my thesis and always I read 'Fourier modes' but I don't understand why is necessary that we work in the Fourier space and what means 'Fourier modes'? Maybe it is a ...
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0answers
25 views

How should be 'time' understood? [duplicate]

Is time an invention or discovery? How to relate time with the gravity via general relativity theory?
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1answer
80 views

The “Many Worlds” description of quantum mechamics and the “Multiverse”

I see a Phys.SE question, asking if the "Many Worlds" and the "Multiverse" ideas are essentially equivalent. This question was quickly squashed with a no answer. I would like to rephrase the question. ...
5
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1answer
550 views

Could Alpha Centauri be made of anti-matter? [duplicate]

How can we tell if a star is made of matter or anti-matter? For example, is there any difference spectroscopically? Or could we tell from cosmic rays? If half of the stars were made of matter and ...
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2answers
81 views

Does the age of the universe depend on the speed of light?

In the Big Bang theory of the universe, does the value of the speed of light enter into the calculation that determines the age of the universe, and if yes, in which way?
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1answer
65 views

Dark matter in the time window between freeze-out and kinetic decoupling

Background After the freeze-out, when all annihilations have stopped, the abundance ($Y=\frac{n}{s}$) of thermal dark matter species no longer changes with time. However, it is still kept in kinetic ...
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3answers
182 views

Is it true that we are probably Boltzmann brains? [duplicate]

I read a post on here about boltzmann brains a few days ago, and got curious on the subject. From the articles I read, it seems that they are saying that we are almost 100 percent likely to be BBs ...
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1answer
29 views

Help wanted on distributions of celestial bodies by mass

I'm looking for help or advice on working out the numbers of bodies in a particular mass range, especially for non-luminous bodies less massive than stars. This is for bodies in interstellar space, ...
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1answer
60 views

Can black holes really evaporate? [duplicate]

Stephen hawkings black hole evaporation theory assumes evaporation of black holes by losing mass through virtual particle radiation. As far as i know, the theory isn't based on mathematical ...
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1answer
67 views

A thought experiment related to the age of the universe

Okay, So, this thought experiment starts with the movie, Interstellar. In which humans went through a wormhole to a system consisting of a Black Hole. They landed on a planet (I don't remember its ...
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1answer
60 views

If two places in the infinite universe have the same state and history, is there a way determine they are different locations? [closed]

I've heard it said that in an infinite universe if you could go far enough in one direction you would end up in a location identical in every aspect to the one you left. Same Earth, same culture, same ...
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1answer
90 views

Why is the age of the universe important?

Age may only be a number, but when it comes to the age of the universe, it's a pretty important one. Why?
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1answer
49 views

The description of a universe with one or two objects

I understand that in general relativity, we describe the universe by the relationship between objects "in it". I quote "in it" because in this model, there is no background where the objects exist. ...
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1answer
33 views

Increasing expansion rate and time dilation

Does the increasing dispersion of matter that occurs as the universe expands impact time (and therefore observed speed) for older/further compared to newer/closer events being observed simultaneously ...
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2answers
195 views

Why does nobody ever consider the possibility that the universe is not smooth?

Disclaimer: I'm not an astronomer, physicist, mathematician, etc. so this is a question from a complete newbie. One of the greatest mysteries of our age is "where is the dark matter?" The universe ...
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0answers
75 views

The answer to this question is not close to the age of the universe, then why do we say it is?

I saw this question in our textbook A great physicist of the century (P.A.M. Dirac) loved playing with numerical values of Fundamental constants of nature. This led him to an interesting ...
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1answer
2k views

What percentage of the mass/energy of the universe is in the form of electromagnetic waves? [duplicate]

Is there any theoretical framework or model that would lead to a prediction, either precise or approximate, about how much or what proportion of the universe's total mass is in the form of photons, or ...
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1answer
44 views

Angular Momentum — AP Physics C

Do all of the planets in our solar system have the same angular momentum? If so, why? I'm not sure if the following is correct: If we could drag a planet radially outward from the sun, then no ...
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2answers
119 views

Is Young Earth universe distinguishable from non-creationist one? [closed]

While talking today about some religious beliefs I was struck by the next question: So what if some super-natural being forks our universe 6,000 years ago, i.e. creates the exact same copy of ...
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2answers
103 views

The limit of the Universe that we'll ever be able to see

After watching Neil Turok's talk at 13:00 - 13:20 he says that the Universe expands and there is a limit beyond which we will never be able to see anything. And that's due to the Dark Matter which (...
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1answer
84 views

Expansion of the universe – impossible for future astronauts to make it back

The universe is rapidly expanding. If future astronauts built a spaceship that could travel close to the speed of light and took off, there is a certain point where they could not make it back again ...
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0answers
40 views

Big crunch scenario, would black holes form?

Imagine a big crunch scenario in which everything (or almost everything as for I have other doubts, eg about entropy...) rolls back. Shouldn't the contracting universe reach a point of density at ...
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0answers
35 views

'My question is about' …the size of the Universe after Inflation [duplicate]

Hitherto, I had assumed, on account of Inflation, "boom" the Universe was "out there" and it has taken light 13.6 BY to get here. However... Quoting the aerospace engineer: In my thinking if the ...
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0answers
40 views

Do Holographic Screens eliminate the need of finding holographic dualities?

There are various models in physics based on the famous holographic principle (https://en.wikipedia.org/wiki/Holographic_principle) This does not always work since in these models we must find a ...
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4answers
103 views

Flat yet finite Universe

I would like to ask you this question: Let's assume that the universe is perfectly flat and big bang happened as described by our theories some 14 billion years ago. Shouldn´t, therefore, the radius ...
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0answers
44 views

Fine tuning of the cosmological and gravitational constants [closed]

Consider the gravitational constant $G$. According to philosopher Robin Collins, cosmologists have been able to determine a region $W_r\subseteq\mathbb{R}$ for which the universe would have permitted ...
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0answers
44 views

Why does Penrose expect visible Hawking Points in CMB?

I've only just started reading into Conformal Cyclic Cosmology. My understanding of the hypothesis is that, with some conformal transformation of space, the very end of the universe at $t=\infty$ ...
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0answers
54 views

Adding up gravitational fields of infinitely many objects?

I'm reading a book introducing gravity and find something I don't understand. Please check the attached image. The sentence in red bracket claims that if one adding up gravitational fields of ...
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1answer
89 views

Is it possible that the universe is not expanding faster than the speed of light? [closed]

What if the speed of light increases proportionately to the expansion of space? Is it possible that light traveling in the medium of space gets to travel faster if the medium is expanding? An analogy ...
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0answers
48 views

Carlo Rovelli: “Is Time’s Arrow Perspectival?”

I came across a paper in which Carlo Rovelli argues that "the entropy we ascribe to the Universe depends on the peculiar coupling between us and the rest of the universe". Rovelli suggests that the ...
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1answer
88 views

Why is the measure problem an issue if the universe is always finite?

As a follow-up from this question, I was wondering how the measure problem applies to the idea of eternal inflation. First, why is it even a problem? As time progresses, more universes are created in ...
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2answers
112 views

How necessary is inflation given the laws of physics?

How deeply is cosmic inflation required for the laws of physics? Is inflation required for a universe remotely like ours, or is it simply a contingent on the starting conditions of the universe? For ...
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2answers
116 views

Is the speed of light in a vacuum constant?

The news site is titled, “Speed of Light May Not Be Constant, Physicists Say.” It talks about how the speed of light in a vacuum is not constant and that estimates of the size of the universe might be ...
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1answer
103 views

Inflation v.s. Big Bang

It is said that that transition or certain quantum rotor field, and “rolling” down into the valley (of some quantum potential), is what causes inflation to come to an end, and create the hot Big Bang. ...
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3answers
111 views

COSMIC ACCELERATION

Please forgive a probably naïve question from a lay person. I understand that galaxies are all moving away from us in every direction we look. And for any given distance the speed is the same. And ...
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2answers
83 views

How will Andromeda collide with Milky Way in spite of Hubble's law? [duplicate]

According to Edwin Hubble our Universe is expanding because he noticed that other galaxies are moving away from us. But then how Andromeda galaxy will collide with Milky Way in ~4.5 billion years, as ...
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0answers
38 views

Why are we in 3 spatial dimensions? [duplicate]

I am very curious to find if there is any reason that why are we in 3 dimensions not in any other like 1D would be hard almost impossible but 2D could have been possible, or what about 4D or in ...
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3answers
88 views

Is the expansion of the universe uniform? Are there parts which are expanding faster than others?

Is the expansion of the universe uniform? Are there parts which are expanding faster than others? The very fabric of the universe is expanding..but is it expanding at a constant rate? If it is not ...
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0answers
26 views

Is it possible that the acceleration of the growth of our universe is driven by the overall gravitational pull of neighboring universes [closed]

I am painfully unequipped when I think about my my knowledge of physics. Still, I am enthralled, as many of us are, by the mysteries and complexity of the universe, both known and unknown. I was ...
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2answers
59 views

Can the universe be seen to have an information entropy of about zero?

I recently made some thoughts about fractals and especially how much information they carry. If we consider for example the Mandelbrot set, we can basically "find" an infinite amount of patterns at ...
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0answers
48 views

If universe is expanding then relative to what? [duplicate]

I am very keen about the fact that universe is expanding.If something expands, it expands relative to something else. If the universe is expanding, then it must expand relative to something else. What ...
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1answer
29 views

Hubble Parameter in terms of Density Parameters

So I have the equation (derived from the Friedman equations with $a(t_0)=1$ and the powers of $a(t)$ quoted)$$H^2 = H_0^2\left(\frac{\Omega_{R_0}}{a^4(t)}+\frac{\Omega_{M_0}}{a^3(t)}+\Omega_{\Lambda_0}...
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1answer
129 views

Question about a mirror Universe

In the article We've seen signs of a mirror-image universe that is touching our own in the popular magazine “New Scientist” (which describes a real scientific theory though, though in a popular form, ...
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0answers
32 views

What does the Hartle-Hawking state imply?

According to a book I read, the Hartle-Hawking state is a wave function that is meant to explain how the universe started. In the book the author explained that according to Hartle and Hawking’s ...
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2answers
65 views

Can we calculate the expansion rate of the Universe at each moment in time?

Do we know the expansion rate of the Universe at each moment of time $t$ of its history? We only measured the expansion rate for a few decades (let's say), which isn't much compared to the age of ...