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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How long would take for other galaxies to be unobservable due to Cosmic Expansion?
I heard that if Universal Expansion continues at some point galaxies will be so separated that a future civilization would have no way to know there are other galaxies, for them the Galaxy they live ...
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If all the black holes in the Universe were combined into one supermassive black hole, what would its diameter be?
I am curious to know if anyone has ever sat down and calculated what the diameter of a black hole would be, in kilometers, if it were to contain all the mass of all the black holes that are currently ...
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Does the holographic principle allow realistically-sized universe simulation?
I was reading about the holographic principle and a question came up to my mind which I can't find an answer to. My understanding is that the holographic principle states that the description volume ...
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The General form of the Friedmann equation written in another way
Using the general form of the Friedmann equation:
$$H^2 =H_0^2(Ω_{m0}(1+z)^3+Ω_{r0}(1+z)^4+Ω_{k0}(1+z)^2+Ω_Λ)$$
and taking $a_0=1$, How can I derive that the Friedmann can be writing in the following ...
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What will happen to the the various fields of the universe after heat death?
What will happen to the various fields like the electromagnetic field, gravitational field etc that make up the universe after heat death? Will they continue existing even after a heat death?
Sorry if ...
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Is it possible for the universe to return to the exact current state if information cannot be lost?
My understanding is that, if information in the universe cannot be lost, it will always be possible (in principle) to tell which prior state of the universe has led to the current state. Is this true ...
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How to know the scale of an extra dimension in units of length?
I’m studying a model with an extra fifth dimension with
a scale factor $b(t)$ . The modified Friedmann-like equations are:
$$
\left[ {\left( {\frac{{\dot a}}{a}} \right)^2 + \left( {\frac{{\dot a}}{a}...
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Is Eddington-Lemaître universe relevant to current cosmology?
The "Modern Cosmology" article in Scholarpedia.org mentions the Eddington-Lemaître universe as an example for "an emergence from a static state" as a possible start of the universe....
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Continuity equation for the estimation of cosmological dimensionless parameters
I want to set up a system of equations to find the dimensionless density parameters $\Omega_i$ as a function of $N=\ln(a)$, from the continuity equation:
$$\dot{\rho_i}+3H(p_i+\rho_i)=0$$
where the ...
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What are the appropriate densities and temperatures of constituents of the universe after reheating?
In slow roll inflation, the inflaton field has to decay into the currently observed constituents of the universe at the end of inflation. This process is called reheating. What are the experimental ...
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Does this paper show that the universe could have come from nothing?
The beginning of the universe is exciting and as Ive read about it I’ve come across a paper that talks about quantum mechanics and how quantum fluctuations can cause space to be created from a meta ...
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How hot is a primordial black hole and does it has angular momentum?
I am fascinated by these elusive phenomenon, they are formed in the early stage of the universe where it is not homogeneous and some regions are very dense enough to undergoes gravitational collapse. ...
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Energy density of particle species in thermal equilibrium
I am reading the book Kolb and Turner "The Early Universe". In the thermodynamics section they mention that the total energy density of different species in equilibrium is
$$\rho=T^4\sum_{\...
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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 ...
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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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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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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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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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Unitarity and Legendre polynomials
Zohar Komargodski in these lecture notes claims:
I don't understand the requirement about unitarity.
Why must residues be expressed in terms of Legendre polynomials?
How does this statement follow ...
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According to Hartle-Hawking state, could we build a sum over all possible metrics (including non-compact ones)?
Physicists Stephen W Hawking and James B Hartle 1 proposed that the universe, in its origins, had no boundary conditions both in space and time.
To do that, they proposed a sum over all compact ...
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What would happen if our galaxy and an identical anti-matter galaxy annihilated each other?
What would happen if every bit of matter in our galaxy annihilated with an anti-matter equivalent at the same time? Such as that stars completely annihilated with identical antimatter stars, planets ...
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Upper bound proton/electron excess in the universe?
There are presumably very nearly equal numbers of protons and electrons.
But is there any bound on their ratio ?
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graph relativistic degrees of freedom
I'm trying to graph the relativistic degrees of freedom, which should look like the figure
And I am trying to guide me with this Phys.SE answer: Number $g(T)$ of relativistic degrees of freedom as a ...
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What's the farthest away from us a photon could theoretically reach?
Assuming the expansion of the universe will be dominated by dark matter, is there a limit to how far a photon could travel in terms of co-moving distance, as the Universe will be expanding faster than ...
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Expanding space outside our galaxy
Some other Galaxy would have its own Hubble sphere. It is possible then that we could be on the edge of someone else’s Hubble sphere. Why is it then we are not experiencing this accelerated expansion ...
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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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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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Was the filament structure of the universe due to the same process as in the continuity equation?
In this long Twitter thread, Dr Phil Metzger suggests that the large-scale filament structure of the universe is due to the same process that determines the continuity equation.
The entire tweet ...
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Spacetime diagram of LCDM
I have a question concerning the LCDM spacetime diagram https://i.stack.imgur.com/Uzjtg.png published on the Physics Forum Stack Exchange website
Can space expand with unlimited speed?
How are the set ...
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From the expansion to temperature and recovering equilibrium number densities from the scale factor
For a particle species of mass $m$, when the temperature $T$ of the Universe is $T>m$, the equilibrium number density $n$ falls as $$n\propto T^3\tag{1}$$ and for $T<m$, it falls as $$n\propto ...
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Entropy and observers
Struggling to understand the stance Bousso takes in his paper, The Entropic Landscape,
https://arxiv.org/abs/1001.1155
Is he saying that because only by observing an increase in available state we ...
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Is the universe a lattice?
My understanding is that cause and effect create a partial order of events in the universe (and that relatively prevents anything stronger).
My understanding is also that it’s generally useless to ...
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Effect of local reference frame and rotation of Universe in CMBR anisotropy
In Kolb and Turner's Early Universe, (see here) it is mentioned that
Variation in the CMBR temperature in different directions is expected due to several effects: the motion of our local reference ...
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Does the virial theorem prohibit dark matter BH's on any size or timescale, or just make them very unlikely?
I've read other Q's on dark matter/black holes, but none seem to be technically specific enough to clarify this point.
I understand so far, that because classic CDM doesn't interact in ways that ...
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How were the particles created from the vacuum state, in the beginning of the universe?
How were the particles created from the vacuum state, in the beginning of the universe?
If the particles are created when some energy is supplied to vacuum state, then how were particles created?
...
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How does the scale of homogeneity and isotropy of the universe change as we go back in time?
This question is an upshot of a previous question asked by me.
The FRW metric of the Universe is based on homogeneity and isotropy of the universe on a length scale of 100 Mpc or larger. If we go ...
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What is meant by "spontaneous creation" in this paper?
I have some questions in regard to the paper "Spontaneous creation of the universe from nothing". If I am not mistaken it is akin to Alexander Vilenkin's proposed cosmological model that has the ...
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The expansion of the universe and its edge
I have understood that the universe is expanding at an accelerated rate. I have also thought, perhaps mistakenly, that this expansion means not that the masses in the universe are moving away from ...
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Are there characteristics that distinguish our universe?
Are there any defining characteristics of our universe that could be different for other universes (or "instances of our universe") that operate under the same laws of physics?
And which, if any, are ...
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Dust mass-loss rate from a massive star given a set of parameters?
I've been looking for examples at how mass-loss rates are determined.
I'm studying a circumstellar dust shell ejected from a Wolf-Rayet star. I have some parameters like, expansion velocity of the ...
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Experiments to resolve dillema between continuity and discrete
Which experiments/experimental methods are suggested to resolve an alternative about the structure of our universe space and time - is it continuous or is it discrete in a very small scale, especially ...
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Looking for more information about these two hypotheses
Big Lurch - This was one I read about in a special issue of Scientific American from 2014. It was explained as
The Big Lurch is a theoretical scientific model suggested as one of the possibilities ...
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What evidence is there of a universe older than 13.8 billion years
I've read an analogy that finding iron-rich galaxies just 900 years after the Big Bang is like finding an old man in a crib in a nursery. We just recently found a supermassive black hole 12 billion ...
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A model that predicts the universe is finite in space and time
A physics professor talked in the YouTube video Cosmological Constant & The End of the Universe - Sixty Symbols about a paper he and his colleagues published. The paper presents a model that ties ...
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According to Penrose, Weyl Tensor=0 is the start/end of the Universe. Must it =0?
If I understand Roger Penrose's theory, as the Universe expands and cools, matter is consumed by black holes. Eventually the black holes collide and evaporate so that there is only energy and the Weyl ...
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CMB radiation and holography
If in principle someone have the exact data of the CMB of our universe and unlimited computability power, to what extent it is possible to determine how the universe should look like? Does it fully ...
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Can universe be a closed manifold?
I had a question at MSE which gave a rise to another question.
Maxwell equations can be written in form $$d\star F = J$$
Then by Stokes theorem we have
$$ \int_U J = \int_U d \star F = \int_{\...
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Total positive charge in the Universe
In their last homework, some of my students miscalculated a charge to be $10^{20}$ C over a squared meter and I was wondering if there was as much positive charges in the entire Universe. It would do ...
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Expanding Universe Balloon Analogy - Anything Similar for Time?
It is difficult to imagine the infiniteness of space and how it itself is expanding rather than the universe expanding into something else. A helpful analogy is that of drawing little dots (...
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Is there an equation that can estimate chances of alien life in the universe?
Seeing how infinite the universe appears and out of all of those stars, planets, galaxies, there must be other life forms. Mathematically, the odds are very good. Is there a mathematical equation to ...
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