New answers tagged

1

To continue this as an addendum, I played around with some of this. It is clear that for a circular orbit that the cosmological constant will only slightly adjust the radius. There will be no change in the radius of the orbit with time. For an elliptical orbit this might be different over a very long period of time. I will continue with this dynamical ...


2

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 ...


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

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)/...


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 ...


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 ...


5

These notes put some numbers on @ACuriousMind 's answer: one needs to be looking at length scales of 100 Mpc and greater for the FLRW metric to be a realistic description of reality. That's a staggering distance, and equivalent to timescales amounting to the whole Mesozoic era, comprising the rise and fall of the Dinosaurs! So one cannot expect the ...


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 ...


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 ...


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

To carry forwards with John Rennie's response let us divide the metric by $dt^2$ $$ \left(\frac{ds}{dt}\right)^2~=~1~-~a(t)\left(\frac{dr}{dt}\right)^2 $$ and with the generalized Lorentz gamma factor $\Gamma~=~\left(\frac{dt}{ds}\right)^2$ means we have $$ \Gamma~=~\frac{1}{\sqrt{1~-~a(t)\left(\frac{dr}{dt}\right)^2}} $$ This factor explodes at $t~=~0$ and ...


3

This is essentially the same as the narrowing of the light cones that happens as you approach the event horizon of a Schwarzschild black hole, and it occurs for the same reason i.e. the coordinate velocity of light tends to zero as you approach the horizon. There is nothing physically interesting in this. It is a result of the coordinates we are using. If ...


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 ...


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

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.


0

It may be early to say it is not a cyclic universe. Everything we see around us (in spite of increasing entropy), appears to be cyclic. See atoms, solar systems, galaxies, clusters etc. Even the rate of expansion of universe has gone through cycles of acceleration and slow down. The last switch from slowed down expansion to accelerated expansion is expected ...


1

First off, let's start with the more common misunderstanding. The Big Bang was not an explosion of any kind. Popular science likes to depict it as an explosion because of the name "Big Bang" and also because it's more visually appealing than what the Big Bang actually was. The actual definition of the Big Bang is a little complicated, but suffice to say it ...



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