The study of the large-scale structure, history, and future of the universe. Cosmology is about asking and answering questions about the "big picture" - the extent, origin, and fate of everything we know.
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1answer
34 views
Questions about the Jeans length
I have a couple of questions about the Jeans length. Suppose the universe has a homogeneous energy density, except there's a spherical region that is overdense.
I understand that if the region is ...
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1answer
226 views
Is antigravity the source of accelerating expansion (dark energy)?
Is antigravity the source of accelerating expansion(dark energy)?
From the observation of 1998, we found that our universe has been continuing accelerating expansion, and the unknown cause for this ...
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7answers
1k views
How many bits are needed to simulate the universe?
This is not the same as: How many bytes can the observable universe store?
The Bekenstein bound tells us how many bits of data can be stored in a space. Using this value, we can determine the ...
16
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3answers
714 views
How do we know Dark Matter isn't simply Neutrinos?
What evidence is there that dark matter isn't one of the known types of neutrinos?
If it were, how would this be measurable?
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0answers
56 views
Are Dark Energy, Dark matter invented concepts to cover up our lack of understanding? [closed]
Dark Energy?, Dark matter? mean two entities that we have no proof of its existence, we do not know what they are and we just invented those names to cover up what we do not know or understand, while ...
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1answer
60 views
Assuming space is infinite can our observable universe be an island amongst an archipelego?
According to recent measurements our observable universe is roughly 93 billion light years in diameter; also it appears (according to WMAP measurements) that spacetime is flat.
Supposing space is ...
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2answers
164 views
Excluding big bang itself, does spacetime have a boundary?
My understanding of big bang cosmology and General Relativity is that both matter and spacetime emerged together (I'm not considering time zero where there was a singularity).
Does this mean that ...
5
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0answers
86 views
Equation of state of cosmic strings and branes
I'm sure these are basic ideas covered in string cosmology or advanced GR, but I've done very little string theory, so I hope you will forgive some elementary questions. I'm just trying to fit some ...
2
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0answers
300 views
Calculation of the non-Gaussity parameter for primordial cosmological perturbations by the ADM Formalism
Maldacena has used the ADM Formalism in one of his papers (http://arxiv.org/abs/astro-ph/0210603) in computing the the three point correlation function (i.e the non-Gaussianity) parameter for ...
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2answers
88 views
Is it possible that universe might not be speeding up expansion?
I'm not sure but I was thinking of galaxies shrinking with time while still moving apart from each other at almost a constant speed or less (i.e: uniform/slightly decelerating expansion). This may ...
4
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2answers
68 views
Cosmological triangle with PLANCK results
Is there an updated version of the cosmological triangle with recent PLANCK results included?
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1answer
79 views
Is the underlying pattern of all dynamical physical processes in the universe actually chaotic? [closed]
After studying and pursuing research interests in chaos theory since the mid-1980s, I have to question whether standard physics (including all physical theories we currently use at all scales of size ...
3
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1answer
111 views
Lookback Time & Age of the Universe Calculations
I try to calculate the age of the universe with the FLRW model:
$$ H(a) = H_0 \sqrt{\Omega_{\mathrm{R},0} \left(\frac{a_0}{a}\right)^4 + \Omega_{\mathrm{M},0} \left(\frac{a_0}{a}\right)^3 + ...
4
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1answer
96 views
Polaris distance correction and implications for cosmological measurements
Polaris has been a guiding light of navigation for centuries. But Polaris also happens to be the closest cepheid variable to our sun. These, together with the type II-A supernovae constitute standard ...
8
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2answers
242 views
Redshifted from what?
We need to know two of the following three to calculate the third: redshifted color, baseline color, and velocity. The velocity is related to the difference between the redshifted color and the ...
1
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3answers
143 views
Can space expand with unlimited speed?
At the beginning, right after the Big Bang, the universe was the size of a coin. One millionth of a second after the universe was the size of the Solar System (acording to ...
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2answers
71 views
If everything in existence were increasing in size at some rate, would we be able to detect it?
Would our eyes observe any changes?
What about electronic measurement devices?
6
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4answers
913 views
Shape of the universe?
What is the exact shape of the universe? I know of the balloon analogy, and the bread with raisins in it. These clarify some points, like how the universe can have no centre, and how it can expand ...
6
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2answers
310 views
Have red shifted photons lost energy and where did it go?
I think the title says it. Did expansion of the universe steal the energy somehow?
6
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3answers
442 views
Is the total energy of the universe constant?
If total energy is conserved just transformed and never newly created, is there a sum of all energies that is constant? Why is it probably not that easy?
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5answers
480 views
Does the amount of gravitational potential energy in the universe increase as it expands?
It seems to me that extra gravitational potential energy is created as the universe expands and the distance between massive objects such as galaxy clusters increases; this implies that energy is not ...
3
votes
2answers
95 views
Is time going backwards beyond the event horizon of a black hole?
For an outside observer the time seems to stop at the event horizon. My intuition suggests, that if it stops there, then it must go backwards inside.
Is this the case?
This question is a followup ...
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3answers
185 views
Could some Red and Blue shifts be the result of light passing through “dark matter”?
As i see it, light behaves in certain ways, as the Double Slit experiement shows,
So when light comes into contact with dark matter, it becomes both a wave and a particle, the wave is bent around the ...
4
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2answers
314 views
Is spacetime an illusion?
In consistent histories, for gauge theories, can the projection operators used in the chains be not gauge invariant?
In quantum gravity, for a projection operator to be gauge invariant means it has ...
6
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4answers
1k views
Did time exist before the creation of matter in the universe?
Does time stretch all the way back for infinity or was there a point when time appears to start in the universe?
I remember reading long ago somewhere that according to one theory time began shortly ...
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8answers
2k views
What are cosmological “firewalls”?
Reading the funny title of this talk, Black Holes and Firewalls, just made me LOL because I have no idea what it is about but a lively imagination :-P (Sorry Raphael Bousso but the title is just too ...
3
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3answers
323 views
References for ADM formalism and cosmological perturbation theory [closed]
What would you consider the best online resources for learning the 3+1 ADM formalism and gauge invariant perturbation theory in cosmology? (Assuming intermediate level GR and QFT familiarity)
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2answers
63 views
proportion of dark matter/energy to other matters/energy at the beginning of the universe
How will the proportion of dark matter/energy to other matters/energy be like at the momenets after the beginning of the universe (standard Big Bang model)?
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2answers
89 views
Does the distance to the cosmic horizon Lorentz-contract? Does the universe Lorentz-contract?
Our universe has a finite size. It is often called the "radius of the universe", or "distance of the cosmic horizon".
If we would fly with relativistic speed at the position of our Earth, would this ...
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2answers
110 views
A question to the theory of multiverse
Guys I couldn't catch a point of multiverse theory..
Theory: If space-time goes on forever, then it must start repeating at some point, because there are a finite number of ways particles can be ...
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0answers
15 views
CMB anisotropies and tightly coupled limit
Sorry if this is a technical question. I am studying the origin of CMB anisotropies and the tightly coupled limit of the Boltzmann equations. We have a fluid composed of ionized electrons and photons.
...
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3answers
80 views
Acceleration in the rate of expanansion of the universe due to weakening gravity?
Could the acceleration in the rate of expansion of the universe be due to the weakening of gravitational forces, as the distance between objects continues to increase?
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2answers
211 views
How is causal patch complementarity compatible with behavior during inflation?
Causal patch complementarity is the conjecture that in de Sitter space with a positive cosmological constant, the states within the causal patch are sufficient to fully describe the universe with the ...
1
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3answers
206 views
Mechanism for the gravitational field generated by photons
This question follows from a schooling I received in this thread.
I figured that photons do not interact with gravity, except when they've spontaneously converted into a particle-antiparticle pair. ...
4
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0answers
34 views
Status of large-scale structure formation within cosmology today
Since the CMB results of the past decade, would it be fair to say that the consensus among cosmologists is that cosmic strings are no longer considered as a (major) source for density perturbations?
...
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3answers
112 views
Cosmological redshift interpretation
Can the cosmological redshift be interpreted as atomic frequencies increasing by the scale factor as the Universe expands?
This explanation seems closer to the truth than the popular idea that a ...
2
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1answer
243 views
Does a complete theory of quantum gravity require anthropic post-selection?
Does a complete theory of quantum gravity require anthropic post-selection? Certainly the black hole complimentarity and causal patch conjectures highlights the essential role of observers, at least ...
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1answer
28 views
Missing step while (classically) deriving Friedmann equation
I'm trying to understand the classical derivation of Friedmann equation but I'm missing one step.
So, I start with accelerations, where $a$ is a scale factor
$\ddot{a}=-\frac{GM}{a^{2}}$
...
2
votes
2answers
171 views
What is the current radius of cosmological event horizon?
Doing some crude calculations (using the value of $H_0$ at this point of time only, since it is time dependent but not distance dependent thanks to Johannes answer) what is the radius of cosmological ...
2
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1answer
80 views
Perfect fluids in cosmology?
In cosmology, it is often assumed that the equation of state of a cosmological fluid is of the form $p=w\rho$. Why is this? Is it the equation of a perfect fluid?
Why does $w=0$ for matter $1/3$ for ...
3
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0answers
147 views
Quantum Entanglement Versus Inflation in the Early Universe?
Quantum entanglement is one of the most fascinating and mysterious phenomena in nature. It needs no interactions, or any sort of exchange for it to take place. It is possible, not against any rules of ...
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1answer
90 views
What is the curvature of an empty universe?
My calculations tell me an empty universe has hyperbolic curvature. Is this correct? If it is, can anyone help me understand why this is intuitively?
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1answer
104 views
Did space and time exist before the Big Bang? [duplicate]
I accept the Big Bang theory. What I can't understand is how there can be a where or when to the Big Bang if space time did not exist prior to it. Did space and time exist prior to the Big Bang?
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1answer
91 views
Inflation and scalar spectral index
I've been reading that the results from the Planck satellite constrain a number called the "scalar spectral index" to be 0.96 rather than 1 at the 5-sigma level.
This is supposed to be big news, but ...
12
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3answers
104 views
What is meant when it is said that the universe is homogeneous and isotropic?
It is sometimes said that the universe is homogeneous and isotropic. What is meant by each of these descriptions? Are they mutually exclusive, or does one require the other? And what implications rise ...
2
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1answer
65 views
How fast did hydrogen atoms travel when they were first formed in the early universe?
I can't seem to find any data on this, is it a known value?
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0answers
37 views
When spacetime expands to the point where galaxy clusters are not observable, will there by any interaction?
It's my understanding that in a few billion years, clusters of galaxies won't be able to directly observe one another due to the expansion of spacetime overcoming gravity between those clusters. ...
1
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1answer
80 views
Ricci scalars for space and spacetime, local and global curvature
If Ricci scalar describes the full spacetime curvature, then what do we mean by $k=0,+1,-1$ being flat, positive and negative curved space?
Is $k$ special version of a constant "3d-Ricci" scalar?
...
5
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2answers
64 views
How do interstellar hydrogen atoms form stars?
I would like to learn the basics about how interstellar matter contracts into stars under the influence of gravity.
Some of my questions:
Let's assume an ideal and infinite large cloud of equally ...
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1answer
68 views
Which was first, energy or matter in the creation of our universe?
Was it the Big Bang or was it something else that gives us our universe in its present condition?
Did it all begin with just pure energy that eventually evolved into simple atoms of matter, that ...
