All Questions
Tagged with cosmology estimation
42
questions
0
votes
0
answers
26
views
Is there an estimate for how many monopoles would be produced in the very early universe?
The question really hits it with this one. I haven't really found some good numbers other than the whole “It'd be so many (per Hubble vol) to recollapse the universe”. I don't know if that comes of as ...
6
votes
4
answers
2k
views
Accelerating Expansion of Universe - Why Not Caused by Radiation?
As I understand it, dark matter and dark energy are used as an 'explanation' for how universe expansion is accelerating; because without it gravity would be expected to cause a long term shrinking.
...
10
votes
1
answer
1k
views
What fraction of the universe's energy is contained in photons?
From each point in the universe, the light of billions of stars, galaxies, supernovae etc. can be detected. So there seems to be a lot of energy/momentum "in flight".
Is it possible to ...
0
votes
0
answers
33
views
What was the size of the observable universe at the matter-dominated era and its mass density?
For instance, at the scale factor $a=0.5$ and matter dominated era, what was the size of the observable universe and matter density?
0
votes
0
answers
26
views
What is the expansion of space of one megaparsec in one year?
suppose we know Hubble's constant. In a hypothetical scenario an astronomer finds and confirms Hubble's law for a galaxy 1 mega parsec away.
Next year, how much distance will the hypothetical ...
3
votes
0
answers
84
views
Why are departures from flat spacetime geometry small on scales smaller than the Hubble radius?
In Chapter 5 of Baumann's cosmology book where he discusses structure formation starting from Newtonian perturbation theory, Baumann mentions at the beginning that
Newtonian gravity is a good ...
1
vote
0
answers
91
views
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 ...
2
votes
1
answer
319
views
Size of the universe 13 billion years ago
When wee look at the sky in opposite directions, we can see early galaxies that were formed about 13 billion years ago. At that time, the distance between two such galaxies at the opposite ends of the ...
1
vote
0
answers
35
views
Could gravitational waves be cosmologicaly 'redshifted'? [duplicate]
Is it posible that gravitational waves detected here on Earth could be cosmologicaly 'redshifted'? So, at emission they had higher frequency then after reaching Earth?
15
votes
2
answers
1k
views
Is CMB slowing down all moving objects in the universe?
1/
Object moving relative to the CMB frame of reference will see the CMB blue shifted where it is heading and red shifted where it came from.
Correct?
2/
The blue photons ahead should have more ...
3
votes
1
answer
619
views
How fast is the edge of the observable universe expanding from the Earth?
If the universe is expanding at 73 kilometers per second per megaparsec, what is the fastest expansion speed that we can observe from Earth? I'm assuming that's the edge of the observable universe ...
1
vote
2
answers
655
views
Calculating the size of the universe
I read the numerous threads here for example:
Lower limit of the size of the Universe? (WMAP)
Determining the size of the universe to calculate its age
Size of the universe
But I would like to try a ...
18
votes
2
answers
4k
views
How long ago was the Universe small enough for interstellar travel?
Currently, even the nearest stars are lightyears away, and impossible to reach in our lifetimes. If space is always expanding, and was once infinitely smaller, then at what point in the past was space ...
0
votes
1
answer
210
views
Demonstration: Integral (discrete summing) of $C_\ell$ has a better variance than one single $C_\ell$
I am working in a cosmological context where I use the $C_{\ell}$ quantities coming from Legendre transformation.
I am faced to a issue to prove the gain that we get by computing the variance of an ...
5
votes
3
answers
585
views
If a CMB photon traveled for 13.7 billion years to reach me, how far away was the source of that CMB photon when it first emitted it?
If a CMB photon traveled for 13.7 billion years (- 374,000 years) to reach me.
How far away was the source of that CMB photon when it first emitted it?
My attempt to solve this question was to use the ...
2
votes
1
answer
609
views
How close can we calculate back in time to the instant of the big bang? (until, presumably we need a theory of quantum gravity?)
I've heard a few times in my 5+ decades that we can only calculate the "state of the universe" (loosely speaking, I don't know any math close to that high) back to very close, but not right ...
1
vote
1
answer
82
views
Is the cosmic event horizon viewed from Andromeda different from the one viewed from Earth?
If the most distant galaxies we see in deep space are in proximity of our cosmic event horizon does it mean that if we were on Andromeda galaxy and looking in the same direction we would see objects ...
6
votes
1
answer
282
views
How long would it take to get to the Hyperion proto-supercluster?
Right now, we set a ship carrying no humans going to the Hyperion supercluster (redshift is $z=2.45$, so around $11$ billion light years from Earth) at a velocity of $c/2$ ($c$ being the speed of ...
1
vote
1
answer
114
views
If Hubble parameter varies with time how much it differs for low distances (present) and deep space (past)? [duplicate]
If deep space is shown as it was in the past how can we calculate this time lagging regarding the Hubble parameter which is a variable of time?
0
votes
1
answer
793
views
What is the largest gravitationally bound structure in the universe?
I understand the galaxy superclusters are not bound due to dark energy. I wonder what matter scale will remain gravitationally bound - galaxy clusters, galaxies, stars - if anything at all, as dark ...
4
votes
1
answer
144
views
Cosmology / Interpretation between credibility/confidence_level with bayesian/frequentist approaches
I try to understand the following article :
testing general relativity from curvature and energy contents at cosmological scale
I don't understand the title of figure 1 :
where it is indicated ...
2
votes
1
answer
281
views
What is the expected size of primordial black holes?
I often see it implied that primordial black holes tend to be small compared to stellar masses;
Since primordial black holes did not form from stellar gravitational collapse, their masses can be ...
1
vote
1
answer
116
views
Very Basic Hubble Constant Question
In an article titled "Gravitational Waves Could Solve Hubble Constant Conundrum"
Data from the cosmic microwave background suggests the universe is currently expanding at a rate of about 41.6 miles (...
1
vote
1
answer
67
views
Is the mean CBM frequency more red-shifted since discovered in the 1960's?
If the universe is uniformly expanding at rate x, is possible to measure that the CBM has increased its red-shift value over time? If the red-shift is a static value, is it possible that the ...
270
votes
5
answers
33k
views
If I sliced the universe in half, would the slice go through a star?
This question is based on a discussion with a 10-year old. So if it is not clear how to interpret certain details, imagine how a 10-year old would interpret them.
This 10-year old does not know about ...
6
votes
1
answer
171
views
At any given point in the universe, are there a huge number of photons crossing paths?
If you're standing in a dark night-time place on the Earth and look up at the sky, you see stars. (About 6000, if you're in a sufficiently dark place). You're seeing photons that have passed though ...
2
votes
0
answers
234
views
How to derive number of nucleons in Universe?
I understand that there are roughly $N=10^{79}$ nucleons in the visible Universe. This number comes from adding up the nucleons of $100$ billion stars in $100$ billion galaxies in the visible Universe ...
3
votes
2
answers
181
views
When must an alien civilization send off a signal that can reach us today?
Given the rate of expansion of the universe and the speed with which galaxies separate: Can a rough estimate be given, at which time T (in its proper time) a civilization that lives on a planet which ...
2
votes
1
answer
166
views
Density of baryons
I wanted to recalculate the number of atoms in the universe according to What paper can I cite for the estimated number of atoms in the observable universe to be lower than $10^{82}$? .
But in the ...
4
votes
2
answers
1k
views
What paper can I cite for the estimated number of atoms in the observable universe to be lower than $10^{82}$?
The title contains the whole question. I am a logician in theoretical computer science and want to use the estimated number of atoms in the observable universe to show what a ridiculous amount of ...
1
vote
1
answer
231
views
Estimation of the entropy of the universe
I heard the following way to estimate the entropy of the universe:
using that the entropy is dominated by photons, in particular the cosmic microwave background radiation, which has a wavelength ...
2
votes
2
answers
36k
views
More atoms in a grain of sand than stars in the observable universe?
in this video https://www.youtube.com/watch?v=AbgcITHmXBI the guys in the end says that there are more atoms in a grain of sand than stars in the (observable) universe.
My estimation with this:
100 ...
-2
votes
1
answer
300
views
How many Planck times would there be from the Big Bang to the Big Rip?
How many Planck times would there be from the Big Bang to the Big Rip? (approx.) Does this number have any numerical significance to mathematics?
If you had a CPU clock which had a timer counting ...
0
votes
1
answer
69
views
When did the universe become lighter than water at 1 g/cm3 [closed]
Did the universe become lighter than water before light could travel through it?
What temperature was it at the time where matter was roughly 1g/cm3?
5
votes
3
answers
344
views
How would we estimate, ahead of time, "the chances" of LIGO spotting black holes colliding in the period that it has been operating? [duplicate]
Can anyone summarize calculations that have been done about the theoretical probability of a detectable black hole collision happening in the observable universe within the time that LIGO has been ...
6
votes
2
answers
13k
views
How many electrons are there in the universe?
Someone on io9 estimated there were about 10^80 electrons in the universe, but I want to ask the Stack Exchange physics community.
8
votes
3
answers
1k
views
What was the density of the universe when it was only the size of our solar system?
What was the density of the universe when it was only the size of our solar system? Did it approach neutron star density? Is it physically correct to even ask such a question?
0
votes
1
answer
663
views
Does the Casimir effect give the correct value for Dark Energy?
My understanding is that the Casimir Effect is caused by vacuum energy. Quantum mechanics (QED) predicts vacuum energy, but gets the value grossly wrong, by a factor of $10^{120}$. On the other hand, ...
21
votes
4
answers
21k
views
How far apart are galaxies on average? If galaxies were the size of peas, how many would be in a cubic meter?
The actual number:
How far apart are galaxies on average?
An attempt to visualize such a thing:
If galaxies were the size of peas, how many would be in a cubic meter?
47
votes
2
answers
41k
views
Dumbed-down explanation how scientists know the number of atoms in the universe?
It is often quoted that the number of atoms in the universe is 10$^{70}$ or 10$^{80}$.
How do scientists determine this number?
And how accurate is it (how strong is the supporting evidences for it)...
4
votes
4
answers
3k
views
How long does it take for expanding space to double in size?
I have been reading about Hubble's constant and trying to make 'sense' of the theory of the expanding Universe. Is is possible that space in the universe expands uniformly? If so, absent of other ...
16
votes
3
answers
8k
views
How many bytes can the observable universe store?
Is the number of states in the Universe countable?
What framework could be used to answer the question in the title?