65
votes
Accepted
Shouldn't very very distant objects appear magnified?
Yes. And they are! This is called the "Angular diameter distance turnaround" (or turnover). In the usual model for expansion $\Lambda CDM$, it is at a redshift of about 1.5 or about 15 ...
- 116k
38
votes
How do we know the laws of physics are the same throughout the universe?
Let's start in the middle:
What is the furthest radius we can prove from earth, with absolute certaintity, that the laws of physics do not vary?
Zero. Proofs are found in mathematics and court ...
- 2,275
38
votes
Accepted
Why didn't the Big Bang produce heavier elements?
I think that your thought process is flawed in that you assume that by drastically increasing the temperature you are guaranteed to get heavy elements. As odd as this may sound, this isn't the case (...
- 15k
34
votes
Why doesn't dark matter clump strongly in the center of galaxies, since it doesn't feel either radiation pressure or the Pauli exclusion effect?
The reason is the fact that dark matter is non collisional. The dark matter particles interact only gravitationally, they feel no pressure, right, but they also feel no drag! No drag, no friction, ...
- 4,371
33
votes
Accepted
Is the universe considered to be flat?
I think the reason you're suffering from conflicting sources is that you're mixing both new and old, out-of-date pieces of information. First off, the book you cited was published in 2001 - 15 years ...
- 15k
28
votes
Accepted
Why was the Cosmic Microwave Background (CMB) released at a blackbody temperature of 3000K rather than 30,000K?
The CMB is produced as the ionisation fraction of hydrogen falls from a high value to a very small value. Contrary to what is written in the Quora answer you may have been misled by, this happens at a ...
- 146k
28
votes
Is there physical evidence to distinguish between the expansion of space and an anthropocentric universe?
I'm looking for some kind of observable evidence (that has been observed, or could be observed in the future) that could falsify one or the other theory.
You seem to be requiring one (very high) ...
- 5,966
27
votes
Accepted
Does the recent news of "ten times more galaxies" imply that there is correspondingly less dark matter?
All Conselice et al. (2016) appear to suggest is that when you look at something like the Hubble deep field, there are many faint (and presumably low mass) galaxies that are not seen. This has ...
- 146k
23
votes
Accepted
How do we know the laws of physics are the same throughout the universe?
Nothing can be proved "with absolute certainty"; that is not how science works.
We adopt a working hypothesis that the constants of nature are exactly that; both constant in time and space. Then we ...
- 146k
22
votes
Does the universe have a celestial north?
No. There is no prefered direction of the universe. The universe is homogenous and isotropic, on the large scale, to the best of our knowledge. This means that there is no "special" ...
- 116k
22
votes
Accepted
What's the issue with Olbers' paradox?
Olber's paradox is - as you state - the phenomenon, that the night sky is dark, but would have to be bright as the sun if the universe was infinite and infinitely old. The unspoken assumption here is ...
- 17.5k
21
votes
Accepted
When will the number of stars be a maximum?
TL; DR Somewhere between now and a few hundred billion years time. (For a co-moving volume) Now read on.
If stellar remnants are included, then the answer is very far in the future indeed, if and ...
- 146k
21
votes
Why isn't the CMB blurred by the blackbodies emitted in the time after atoms first formed?
It is. They're called galaxies and stars and we see them against the cosmic microwave background. However, they aren't (all) cooling because of the energy sources they contain.
But perhaps that isn't ...
- 146k
20
votes
Accepted
What equation tells you how far in space you can go from a point and return?
Preparation
Let $a(t)$ be the cosmic expansion factor. Let $x$ be a comoving coordinate, so two objects (like galaxies) that are separating solely due to the expansion of the universe have a constant ...
- 4,260
20
votes
Why is WR124 not brighter?
It's 7.7 times the absolute temperature of the Sun and if it is 12 times the radius.
The luminosity will therefore be $(7.7)^4\times 12^2 = 515000$ times that of the Sun.
Close enough, given you haven'...
- 146k
18
votes
Accepted
Why do people choose 0.2 as the value of linking length in the friends-of-friends algorithm?
The friends-of-friends (FOF) algorithm (Huchra & Geller 1982; Press & Davis 1982, Davis et al. 1985) for finding groups of particles can be used for various numerical problems, not just ...
- 37k
18
votes
Accepted
In 2023, is space still thought to be "filled with a network of wormholes"?
It was never thought to be the case. A fuller quote:
So, by this analogy a black hole is a kind of bottomless pit.
What would happen if you fell in? Assuming you could survive the gravitational ...
- 116k
18
votes
What would evidence of stars and galaxies significantly older than 13.8 billion years old look like? In what parts of space has it not been found?
Assuming the cosmological principle still holds, then we might expect that your own galaxy and those around us should contain evidence of stars/objects older than 13.8 billion years old.
These might ...
- 146k
16
votes
Accepted
Latest cosmological parameters
Cosmological parameters are measured in a variety of ways, and their values will depend on which measurements you trust the most. The paper you link to (Planck Collaboration et al. 2016) with the 2015 ...
- 37k
16
votes
Accepted
Understanding The Turnover Point of Angular Diameter Distance
On the one hand an object spans a smaller angle the farther away it is, as expected. On the other hand, due to the expansion of the Universe and the finite speed of light, very distant objects were ...
- 37k
16
votes
How can gravitational lensing makes a quasar appear brighter?
The quasar gives out light in all directions. The light spreads out in space. Only a very small amount of that light would be pointed exactly in the direction of your telescope.
But if a large galaxy ...
- 116k
16
votes
Accepted
How is the Hubble constant determined from gravitational waves?
If you measure the gravitational waveform from an inspiralling binary, you can at any point measure the amplitude, instantaneous frequency and the rate of change of frequency. The last two give you ...
- 146k
16
votes
What equation tells you how far in space you can go from a point and return?
sten's answer is excellent and beautifully analytical; I just wanted to illustrate the journey and show that neglecting matter (and radiation) is a good approximation.
Spacetime diagram
To do this, I ...
- 37k
15
votes
Accepted
Which came first: Galaxies <=> Stars <=> Planets?
The structure we see in the Universe has formed from the gravitational collapse of the matter that was once an almost smooth density field of gas ("baryons") and dark matter$^1$. The word &...
- 37k
15
votes
Accepted
Why do we continue to find a discrepancy in the Hubble Constant?
To make a long story short, the measurements from Planck and the Hubble Space Telescope disagree, and the reason behind this isn't known.
First, let's look at the values with the uncertainties. We ...
- 36k
15
votes
Accepted
Scientific evidence against CMB originating from “Oort Soup”?
The idea that belts or spheres of dust might be responsible for (some) microwave emission is not crazy. Indeed we know that dust does emit microwaves and indeed, the contribution of such dust has to ...
- 146k
15
votes
Accepted
A Sky Map of Gravity - what would it look like?
The rough answer is: just like the sun makes it hard or impossible to see planets and stars during the day, it dominates the gravity sky.
But there are interesting patterns there if we view the sky ...
- 14k
15
votes
Why did astronomers believe most or all stellar black holes had masses no greater than 15 solar masses?
The so-called 'mass gaps' for black holes, according to theoretical models, are between 2-5 solar masses and 50 to 150 solar masses. (Actually, I have read that there is no good theoretical reason for ...
- 4,439
15
votes
Accepted
What happened to the reemitted photons during recombination?
There should indeed be emission lines at the appropriately redshifted frequencies. However, they are going to be incredibly faint and diluted because the ratio of photons to baryons at the epoch of ...
- 146k
Only top scored, non community-wiki answers of a minimum length are eligible