# Tag Info

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How about space? I understand a standard assumption in cosmology is that the universe is spatially infinite in extent. Of course whether it actually is - is an entirely separate question. For how can you measure something to be infinite? It maybe that some parameter is inversely related to that something. And you could measure that value to be zero. In ...

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This is a philosophical question so here is a philosophical answer. The scientific method in based on repeated observations and experiment. The whole science is just a collectivist instrument of acquiring knowledge. Being an instrument, it has its own limitations. Among them are: The tools employed by science are built by humans. As such, all tools use ...

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No (understandable/explainable) physical quantity could be infinite. "Infinity" is is physically very vague. When we say something is "infinite", it almost means we're throwing our hands up in despair that we can't explain something, or that quantity doesn't make sense in some particular framework. The whole point of physical quantities (observables) is to ...

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Here is a list of actual infinities in physics, mentioning Real Numbers. For instance, a set of all possible distances, this is a set with infinite cardinality, and thus actually infinite. Even the same distance, from infinitude of possible observers, is infinitely large or distance between relatively moving objects has infinity of values, taken at ...

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First of all, galaxies don't shrink. If our own galaxy were shrinking, then we would be moving towards our galactic centre, and we would observe a blueshift in that direction. Second, the accelerated expansion can be determined from the relation between redshift and brightness of distant supernovae. Neither redshift nor brightness would be affected by ...

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I don't see any logical connection to accelerating expansion. If shrinking of galaxies could explain away the acceleration of the expansion, then it could also explain away the expansion itself. Regardless of whether we're talking about expansion or acceleration of expansion, the effect isn't measured by watching the apparent sizes of galaxies get smaller ...

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There are quite a few common misconceptions about the expansion of the universe, even among professional physicists. I will try to clarify a few of these issues; for more information, I highly recommend the article "Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe" from Tamara M. Davis and ...

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The answer is the energy goes into the gravitational field. If you take the simplest case of a spatially flat homogeneous cosmology with no cosmological constant then the equation for energy in an expanding volume $V(t) = a(t)^3$ is $E = Mc^2 + \frac{P}{a} - \frac{3a}{\kappa} (\frac{da}{dt})^2 = 0$ $M$ is the fixed mass of cold matter in the volume, ...

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You didn't really state what do you imagine when asking your question... Depending on which level do you want our things to change their size. Here are a few options: The atoms could increase distance between themseves We'd notice this for sure, because things would became larger, but they'd keep their original weight All particles (proton, neutron, ...

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Depends. If you simple assume matter growing we would see the distance between the surfaces of celestial bodies diminishing. Given that we regularly monitor the distance between the surfaces of the Earth and Moon by laser ranging to accuracies of less than one cm (which means less than one part in $10^8$ over the time the project has been running). This is ...

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The problem with this question is that gravitational potential energy between massive objects is a Newtonian concept but the question of energy conservation in cosmology can only be discussed properly in terms in general relativity. The general answer is that energy is always conserved if you take into account the energy in the gravitational field as well ...

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How do we know the universe is expanding and the speed of light isn't slowing instead (thanks to innisfree for the idea of where to look.) From wikipedia By the 1990s and on into the twenty-first century, a number of falsifying observations have shown that "tired light" hypotheses are not viable explanations for cosmological redshifts.[2] For ...

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Astronomers know about this trouble, and stick with what they can measure. For distant galaxies, quasars, cosmic background radiation, etc. they use only the redshift, the "z" value. This is defined by the measured wavelength and the known laboratory value - assuming any spectral emission and absorption lines are correctly identified. To say anything about ...

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The difficulties are in some ways worse than you imagined and in other ways not as bad. The difficulties are worse in the sense that when we're dealing with distant galaxies, we need general relativity, not just special relativity. General relativity does not even have a well-defined notion of a global frame of reference, so it doesn't offer a uniquely ...

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The first thing to note is that most accurate distance measurements are at about the 5% level (except for a few exceptions), but 50% is more common - and often satisfactory. Length contraction is not an issue for two reasons. First, relative velocities between us and entire galaxies are generally small1 (especially due to 'peculiar velocities'). Second, ...

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If there were a matter-antimatter boundary, we would observe gamma radiation from the annihilation events on the boundaries. We do not observe such radiation events.

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The expansion of the universe is (approximately) described by the FLRW metric. The Wikipedia article I've linked gives lots of gory details, but the key result we need is the dependance on the acceleration on the density and pressure: $$\frac{\ddot{a}}{a} = -\frac{4\pi G}{3}\left( \rho + \frac{3p}{c^2} \right) + \frac{\Delta c^2}{3}$$ If there is no dark ...

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I guess that you are imagining an expansive force accelerating the Universe versus gravity pulling the Universe together, and that if somehow gravity were weaker, the expansive force would win. That is not the correct picture. In popular models, the accelerating Universe is caused by gravity, because of a vacuum energy with negative pressure (see dark ...

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I think the term acceleration implies a change in direction of the net forces acting on galaxies on the large scale. Gravity (regardless of magnitude) acts to collect the galaxies, while the acceleration acts in the opposite direction.

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The dark matter is known (from galactic rotation curves and microlensing) to clump around galaxies and to mass considerably more than the visible matter. This contradicts your hypothesis, so no.

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It's both effects combined. They are not in competition. You get a Lorentz time dilation (i.e. frequencies are slowed) from the relative motion and also the `stretching' of the wavelength (i.e. an additional decrease in frequency) due to the expansion of the Universe. They are separate effects and they combine together.

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Yes, indeed. "cosmological redshift CAN be interpreted as atomic frequencies increasing by the scale factor as the Universe" evolves (NOT expanding). Your statement correspond the interpretation that the 'space is not expanding' but instead the ruler (always made of atoms) shrink as time goes by, giving an apparent increase of the space. Space expansion ...

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"Metric expansion only occurs with proper distances (between events at the same cosmological time)" Why would this effect disappear when you integrate over paths in space-time (e.g. photon trajectories)? The redshifting of light by spacetime geometry is an observational fact, not an ad hoc conjecture. "Can the cosmological redshift be interpreted ...

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