New answers tagged

1

How can we look into the past? Light has a fixed velocity of almost 300.000 meters per second. Sunlight takes about 8 minutes to reach us. So we see the sun always 8 minutes ago. As the other answer says, stars are much further away and it takes light that much longer to reach us. How do we know how far away the stars are? There are various methods that ...


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Stars are very far away. So light takes a while to get from stars to you. The light arriving now shows you what the stars looked like when the light left. It is like getting a letter from a far away friend. The letter took a few days to arrive. It has news from a few days ago.


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You go outside at night and you look at the sky. That's the universe and that's the past streaming in on you. With your own eyes, of course, you can't see much farther than approx. 2.5 million years back - the Andromeda galaxy is easily seen, even though it's not as pretty as in astrophotographs: https://en.wikipedia.org/wiki/Andromeda_Galaxy. What you will ...


0

No, the age of the universe doesn't depend on the observer. What depends on the observer is the "perceived" time that has passed since the Big Bang. What you are asking is if the conformal time and the age of the universe are the same and the answer is negative as you can see in that link.


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Given a few plausible assumptions about the universe its spacetime geometry is described by a solution to the Einstein equations called the FLRW metric. If we know the densities of various types of matter/energy present, e.g. photons/matter/dark energy/anything else, then we can calculate how the expansion of the universe varies with time. Generally ...


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I don't know if negative pressure (but see my added edit below) , more importantly there is a theory of inflation, and some good evidence for it. It was caused by a yet unknown inflation field, with its parameters somewhat matching what the cosmic microwave background (CMB) measurements show. [edit added: The field is a quantum field that rolled from a high ...


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Pocket universes have arisen in different theories. Just to name you two, one is alan Gut's inflationary theory idea that Eternal inflation produces pocket universes with all physically allowed vacua and histories. Another is that from sean carroll, who claims that inside every black hole there is an entirely new universe.


3

You must not really have looked hard enough. They are the same phenomenon The Big Freeze, which is also known as the Heat Death, is one of the possible scenarios predicted by scientists in which the Universe may end. It is a direct consequence of an ever expanding universe. The most telling evidences, such as those that indicate an increasing rate of ...


0

Absolute location, absolute direction, and absolute time are in-explainable with current physics. Therefore absolute speed/velocity does not make sense either. The absolute point where all the expanded matter was concentrated at/before the big bang, may pertain to some location within universe. Even if the point itself expanded, the original reference still ...


2

So, there are several possible ways the universe could be baryon symmetric: A region of the universe where antimatter dominates. There is a problem with this theory, though - 30 years' worth of scientific research has calculated just how far away this type of region would have to be, and from these calculations it is considered very unlikely that any part ...


1

For the case $\Omega_M + \Omega_\Lambda = 1 = \Omega_\text{total}$, which is a good approximation for $t \gtrsim 10^8 \; \text{yr}$, an explicit formula for a(t) is $$a(t) = \left[ \frac{\Omega_{M,0}}{\Omega_{\Lambda,0}} \sinh^2 \left( \frac32 \sqrt{\Omega_{\Lambda,0}} \, H_0 \, t \right) \right]^{\frac13}$$ or, plugging in numbers from Pulsar's answer, $$...


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The idea of a "Zero-Energy Universe" is a theory held by a limited number of scientists. There are several stackexchange question that expand on the theory and may help you. Zero energy universe Total energy of the Universe


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This is known as the "Heat Death" of the Universe which is based on the Second Law of Thermodynamics which says that Entropy will increase in an isolated system (and, we assume the Universe is isolated). So, as Entropy increases over time then ultimately every part of the Universe will be at the same temperature and same level of maximum Entropy such that ...


0

That is not quite that simple. There is theory and measurements, and it places some constraints. There is more. First, if the universe is infinite now it was infinite at the Big Bang. You can have an infinite universe and have it all in the spacetime at the Big Bang. It does not grow to be infinite, it either is or is not. (Ignoring multiple dimensional ...


2

You need to learn to use mathematics to tackle such problems. If you try to do without math using only your intuition then you'll make many hidden assumptions that may not be valid. Physicists who understand some theory well enough can get away with using their intuition, but then that intuition is based on a rigorous mathematical understanding of the theory....


1

Infinity is a mathematical concept, as well as the concept of variables describing dimensions. Physics is about observations, either in the laboratory or of the cosmos, which are fitted with mathematical models. It started with the geocentric system, became the heliocentric system and then the realization that the galaxy is composed out of sun like stars, ...


0

If "open" means nonpositive spatial curvature (which is the usual meaning), then no. The experimentally measured spatial curvature is consistent with zero, but the error margin includes values on both sides of zero. There's no conflict in the Friedmann equations between accelerating expansion and positive spatial curvature. If "open" means "never ...


2

That is the case. The conformal transformation of the metric $g_{ab}~\rightarrow~\Omega^2g_{ab}$ transforms the line element as $$ ds^2~\rightarrow~ds^2~=~\Omega^2(du^2~-~dx^2~-~dy^2~-~dz^2). $$ Then for the case that $du~=~\Omega^{-1}dt$ we have the metric $$ ds^2~=~dt^2~-~\Omega^2(dx^2~+~dy^2~+~dz^2), $$ that is the de Sitter spacetime metric for $\Omega^2~...


0

The universe, by definition, is all there is. So there is nothing outside it, not even space, since if you extrapolate the present expansion of the universe back far enough in time, you end up with a tiny space, just after the big bang occurred. So, from a personal perspective, I ignore the ordinary definition of the word space, as in the "space between New ...


1

The noboundary condition means there is no boundary that marks the end of space or time. With respect to time one might think of the lines of longitude on a globe as representing the time direction at different point in a spatial manifold modeled as the lines of latitude. As one looks further to the north, which is the big bang that eventually you look north ...


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My essay Misconceptions about Virtual Particles explains in detail the difference between the popular science view of virtual particles popping in and out of existence and the physical science view of virtual particles as useful (but literally virtual) imagery for talking about complicated integrals for the computation of scattering amplitudes. From the ...


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Space is the canvas on which the Universe is painted. Space is created when a Higgs wave (dark energy) intersects dark matter. When this happens, three things are created. Matter, energy and a whole lot of SPACE. Space has two known properties. First, Dr. E. tells us it can be warped. Second, I tell you it cannot be compressed. That being said, this reaction ...


1

Sticking with the sphere analogy, first remember that in this analogy, the Universe is a shell, i.e. only the points on the surface of the sphere exist in the Universe, not points inside or outside. If the Universe has a spherical geometry, then the centre would be the centre of this sphere, which is not in the Universe anymore (which is why one would say ...



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