New answers tagged

1

Your question has some bearing on what some people interpret erroneously as the source of the Pioneer anomaly. As some people point out there is some issue with what happens with a solar system in a galaxy. Really the influence of the cosmological constant is most likely to occur on that scale instead of a stellar system of planets. I will set this up some ...


3

Is there any other scientific theory that proves that big bang is the origin of time ? Here is a gross misunderstanding of what a scientific theory is. A scientific theory can never be proven. It is successful if it fits data and observations, then one says it is validated, and if its predictions are always validated. An invalid prediction requires drastic ...


0

Studying big bang itself gives a lot of evidences to believe in it.however redshift ,given by Edwin Hubble , is one of the major theory that supports it by giving evidence of expanding universe.cosmic wave background is also a big point to support it.


0

@knzhou is right in his comment. Light cannot escape from a black hole (BH) because gravity causes a high enough curvature that its paths (lightlike geodesics) outwards all become tangential to the horizon at the Horizon. They don't have to interact with anything, shooting them as straight out from inside the horizon as possible they simply cannot overcome ...


0

We know so little about quantum gravity that there's very little (okay, nothing) about the Planck epoch that we can say with confidence, but it's believed that a semiclassical GR description of the Big Bang initial singularity would be that of a naked singularity, so nothing escaped from behind an event horizon.


0

As a star’s fusion fuel runs out the force of gravity takes over causing the star to collapse on itself. The star is rotating prior to the collapse and as it collapses this rotation at the core increases expediently. As the speed of the matter increases and gets closer to the speed of light, it's mass increases as well further fueling the collapse and in ...


0

The fancy word for a Universe that expands equally in every direction is an isotropic Universe, and one that expands at different rates in different directions is anisotropic. The usual assumption is that the expansion rate is the same in all directions (i.e., the Universe is isotropic); this is the standard Freidman-Robertson-Walker metric for an ...


0

We believe that the universe expand in every direction evenly. Even-if there's any unevenness, it's hard to see, and will only be clear at very very large scales. Some people have combed the CMB (cosmic microwave background) and argue that there's maybe some evidence that things aren't perfectly even, but it's not really clear. Right now it really looks like ...


0

Short answer: Yes Explanation: The answer to this question is something well documented in astrophysics. The "Size" of a universe is modeled by metaphorical expanding fluids known as the Freidman Equations. These equations say that from a singular point, the universe will expand at rates according to the travel of its components: energy and matter, for ...


2

It is an assumption that the universe expands evenly in all directions, and the experimental evidence so far confirms the assumption. Our mathematical description of the expanding universe is based on the assumption that on a very large scale the universe is homogeneous and isotropic, which basically means it's the same everywhere and in all directions. ...


1

See the lookback time to redshift relation in https://en.m.wikipedia.org/wiki/Redshift You can ignore inflation if you get redshifts, temperatures and universe size (radius, scale) ratios between now and times in the past after inflation. For recombination the relations of 1+z to the scale ratios and temperature ratios are linear and direct . So for T(then)/...


1

Our universe looks not only isotropic (the same in all directions) but also homogeneous (the same at each x, y, z at any one time). The fact that our position is not unique is not a principle, it is determined to be so from astrophysical and cosmological observations. Of course, the meaning is that these are so for cosmological distances, i.e., in the large, ...


0

The webpage you linked to is remarkable, to say the least. It contains a whole chain of errors, producing a wrong result that the authors mistakenly interpret as accurate. Based on what I've seen on that page alone, I wouldn't trust anything else from that article. So, first of all, Wolfram Alpha is correct, and I applaud you for checking their calculations....


4

Because when we look around, we see that things are, on large scales, the same in all directions. This would be true in such a model only if we were at the centre of this system. That seems ludicrously unlikely: what are the chances that we are lucky enough to be at the one place in the universe where everything looks the same in all directions? So ...


2

One talks about the size of the universe in the context of a model where spacetime is foliated by three-dimensional spacelike leaves. The "size of the universe" means the size of one of those leaves, not of all spacetime. For example: If you imagine spacetime to be filled with galaxies, the worldlines of those galaxies give a preferred global time ...


1

No one knows. One other thing ... the big bang hypothesis does not explain the origin of the universe (although I recognize that you used the word "formation", not "origin", but I'm not sure what exactly you meant by "formation"). It explains what happens after a certain epoch in our universe's history. What happens before that is completely unknown. ...


2

In "Adventures in Friedmann cosmology: A detailed expansion of the cosmological Friedmann equations" by Robert J. Nemiroff and Bijunath Patla in the American Journal of Physics volume 76, on page 265 (2008); http://dx.doi.org/10.1119/1.2830536 the authors call them "cosmic strings" But this is in the context of cosmology, so its for a universe that on very ...


2

The answer by @peterh is accurate on the factual information about the Einstein Field Equations and that it describes how the matter distribution affects spacetime. There is more that may be added that hopefully will help understand more of it. First, just to be totally clear, gravity as described by GR (general relativity, through Einsteins Field ...


2

No, it depends on the metric of the Universe described by the Friedmann model. It is a general relativistic theory. In GR, gravity is not a force. Instead, there is actually two equations: The Einstein Field Equations, describing how the distribution of matter affects the geometry of the spacetime, There is also equations showing how matter moves in the ...


2

As is discussed in this answer, the "rest frame" of the cosmic neutrino background would be very similar to that defined by the cosmic microwave background if neutrinos were very light (say $<0.1$ eV). The Sun would be moving with respect to this frame at around 370 km/s. But if neutrinos were more massive(say getting on for 1-2 eV) then they are ...


7

I just want to add something to the answers above, because I did not understand this initially. As far as clustering goes, the crucial point is how slow the neutrinos are travelling with respect to the characteristic escape velocities of galaxies (600 km/s) and clusters (2000 km/s). If you assume a rest mass of 0.1 eV, use the 1.95K temperature and the ...


3

As explained in this paper, the dominant effect is due to gravitational interactions, which can yield overdensities up to a factor of $10^3$ for neutrino masses of the order of 1 eV. The clustering of relic neutrinos can be modeled well using the collision free Boltzmann equation (Vlasov equation) where the densities evolve under the influence of the ...


8

I thought about this a little more since I prompted your question and there are a couple of other complications worth noting. This may be more like a comment than an answer, but it's too long for a comment. First, and directly addressing your question: the cosmic neutrino background is already present before the gravitational well of the star forms. If ...


0

The FLRW energy equation $$ \left(\frac{\dot a}{a}\right)^2~=~\frac{8\pi G\rho}{3c^2} $$ gives a solution for the scale factor $a(t)~=~a_0(t)exp(t~\sqrt{\frac{8\pi G\rho}{3c^2}})$. We have here that the density $\rho$ is the density of energy in the vacuum of space. This is most often thought of as due to the zero point energy of quantum mechanics. The ...


8

It requires that they lose energy somehow (to drop hyperbolic orbits into periodic ones). There are two basic mechanisms available: gravitational scattering and weak scattering. In both cases we expect the interaction to be elastic, but that doesn't mean the neutrino has as much kinetic energy in the star's frame afterward the interaction as before: it ...


1

The other answers seems to answer most of your questions, but I think one confusion remains: The speed of light as a maximum speed in the Universe (which is not the case). First off, redshift doesn't go to infinity for objects receding at $v = c$. We easily see galaxies recede at superluminal velocities. In fact, this is the case for all galaxies with a ...


3

I think there is no common explanation for this. Some people try to build some theories about that, but nobody can prove them. Every "beyond standard model" theory probably has it's own explanation for dark energy. One of them is still pretty interesting at least. I remember that Stanley Brodsky talked about that during the NED/TURIC meeting in 2014. He ...


1

Heather is right and it is not much more complex than that. Except you might need to follow the math to understand it. Dodelson certainly has the math. Light goes at c. Period. If you want to find the geodesics of light you set the metric ds^2 = 0. But space itself expands, and it can expand at any spee, it is not a particle or wave or object, it is just ...


1

The problem with the assumptions in your second paragraph is that space is moving, not the galaxies. Space itself can travel faster than the speed of light - that is not forbidden by general relativity. The speed of light as a constant therefore still holds, removing the implications you bring up. As an analogy, imagine you have a coordinate grid, and you ...


6

Your post reminds me of this paper by Wigmans (also via arXiv) which I learned about during a colloquium in the distant past. Wigmans points out a narrow and interesting region in the parameter space for neutrinos in the mass region 0.1-1 eV. A longer paper (arXiv) by the same author; you'll enjoy mining citations. Such neutrinos, redshifted to the CνB ...


12

As far as theory goes, the Cosmic Neutrino Background (CvB) was created within the first second after the Big Bang, when neutrinos decoupled from other matter. Nevertheless, while the universe was still hot neutrinos stayed in thermal equilibrium with photons. Neutrinos and photons shared a common temperature until the universe cooled down to a point where ...


1

Start the definition of the deceleration parameter: $$\begin{align} q_o &= -\frac{\ddot{a}a}{\dot{a}^2} \\ &= -\frac{\ddot{a}}{a}\left(\frac{a}{\dot{a}}\right)^2 \\ &= -\frac{\ddot{a}}{a}\frac{1}{H^2} \\ &= \left( \frac{4\pi G}{3}(\rho + \frac{3p}{c^2}) - \frac{\Lambda}{3} \right) \frac{1}{H^2}\\ \end{align}$$ We are ...


0

In a nutshell, no. General relativity says that objects with mass cannot travel faster than the speed of light. Space itself can travel faster than the speed of light because it doesn't have mass. However, space isn't moving, it is stretching. If you imagine the Milky Way and other galaxies are on a coordinate plane, the proportions between the galaxies are ...


0

Short answer, no. Relativity "forbids" massive matter moving at (or faster than) the speed of light within spacetime, but the recession of distant galaxies is due to the expansion of spacetime itself. As an analogy, put some points on a graph drawn on a rubber sheet, then stretch the rubber sheet. The points will move away from each other, without actually ...


3

This whole equation, as you'll note, provides the basic rate of expansion. It's true that the $\frac{8\pi G\rho}{3}$ term decelerates expansion, however this is mostly because both $\rho$ and $\frac{1}{a^2}$ fall off as $a$ increases. Even with $\lambda$ positive and small, it remains constant, which means that as $\rho$ and $\frac{1}{a^2}$ drop to zero, the ...


1

The reason that energy is usually conserved in most contexts is that Noether's theorem guarantees that energy is conserved in systems with time translational invariance. But the metric of the universe as a whole is (approximately) the Friedmann–Lemaître–Robertson–Walker metric, which does not have time translational invariance (more precisely, there does ...


1

Your question is very broad, and actually popular science books about string theory, particle physics or cosmology are pretty much obliged to write about the subject in much greater depth than here. Anyway here's my short attempt starting with the quantum scale and using QM rather than QFT. We have plenty of equations, all of which incorporate the Planck ...


1

A few million to about 30 million years from now. I.e., we'd be able to measure a change in the previously observed redshifts for galaxies a few to about 10 Mega parsecs away. A parsec is a little more than 3 light years. The reason is that closer in galaxies are in our local group or cluster, and we are to a great extent gravitationally bounded to them. ...


2

The issue is that concepts like distance or relative velocity are problematic in curved spacetimes. Personally, I'd stress that the meaning of metric expansion of space in FLRW cosmology is that matter is distributed in homogeneous spacelike layers of constant age, and the distance as evaluated within these layers between any two particles of same age ...


0

The similarity of time and space is limited to Lorentz symmetry. Beyond Lorentz symmetry, the time dimension cannot be assimilated to space dimensions. Within spacetime, time is not intrinsically curved: Any observed time corresponds to the proper time of a clock, and the clock is always counting straightforward, even if according to our coordinates we may ...


0

Two or more timelike dimensions is a situation that is difficult if not impossible to reconcile with the notion of causality. Suppose you want to think of a five dimensional universe with three spatial and two time dimensions. What you mean then is the metric has a $(2,\,3)$ signature, which means that at each point Riemann normal co-ordinates centered at ...


0

Time is the perception of the order of change in 3 dimensions. Time does not exists, only 3 dimensions of space exists in reality. Time is not a 4th dimension. I think your question highlights the damage done to innocent brains in high schools. Small wonder we do not have enough smart physics students, their brains get destroyed with false assumptions ...


1

The whole premise of the paper is wrong. Citing Synge in 1960 is irrelevant, the Big Bang was questioned to some extent, by a minority of physicists, until the cosmic microwave background was discovered in 1965. The first citation is a diatribe, in arxiv, and the reference to Weinberg saying there was no space expansion was not cited - Weinberg wrote his ...


1

A simple addition to the right answer from @Countto10. So they have Doppler shifts. the question might be, for cosmic microwave background photons, that red shift is with respect to what? The answer is that it is with respect to the so called comoving frame, the frame of reference which is (in a way) the frame of reference expanding with the universe - ...


2

You need to concentrate on the observer making the measurements. Photons carry energy, but they don't lose energy just because they travel. The "loss" of energy is not the cause of the redshift, only if the photon scatters off something will it lose energy. However, not all observers will agree that photon has the same amount of energy. Assume you are in ...


1

As a completely speculative answer, I would say: 1) According to our current theories OUR time did start with the bigbang. I say our time, because it is plausible to think of other parallel universes with their own laws and times. Not even so, but no law forbids that other universes have more than one time dimension. 2) You have two possible answers, ...


0

Actually , time is not absolute ... Newton first had stated that time is aboslute (i.e. it was there forever , even before big bang) . But by his theory of relativity , state that it was relative . But he didn't mean to say that it started before bigbang , instead he tried to say that time could be interfered or altered by some ways which can include too ...


-3

Veritasium was wrong. Don't let a wrong idea by him drive you crazy. His big mistake is that he thinks speed of light is constant even if spacetime expands. That is false. Speed of light (photon) is affected the same way as of speed of maters if spacetime expands.


5

We know some physics that occurred with the early universe up to periods of time close to the big bang. The actual moment of the big bang is not known at least empirically. That moment did not occur at point, but was a process where the spacetime of the observable universe emerged. This was probably a bubble nucleation event similar to that proposed by ...


18

This seems to be a common misconception about the big bang. At present our theories can only suggest what happened AFTER the "bang". We cannot formulate what occurred AT the singularity with our current knowledge of physics. At a small neighborhood around a spacetime singularity quantum gravity becomes important and we simply have no clue at present how ...



Top 50 recent answers are included