In the future it will, when the positive curvature induced by mass/energy is overcome by the negative one of dark energy. This stretching doesn't happen in the galaxies because of the positive warping of gravity. Why is the wavelength of a photon stretching? Because the parts of the excited field accompanying it (a photon is just an excitation of a field) are moving away from each other if the photon travels towards us. The fabric of space itself may still be expanding everywhere, but it doesn't have a measurable effect on every object. The reason for this is subtle, and is related to the fact that the expansion itself isn't a force, but rather a rate. Despite a gravitational pull that's more than a thousand times as powerful as our own.īecause the expansion of the universe only has any effect where another force (gravitational, electromagnetic or nuclear) hasn't yet overcome it. Milky Way's nearest large galaxy cluster, the Virgo cluster, at 50 million light years away, will never pull us into it. Earth will revolve around the Sun at the same orbital distance, Earth itself will remain the same size, and the atoms making up everything on it will not expand. The Milky Way and all the local group galaxies will stay bound together, eventually merging together under their own gravity. The superclusters of the universe, populated with galaxies and stretching for over a billion light years are being stretched and pulled apart by the universe's expansion. The farther away an object is from another, the more "stretching" occurs. The math tells about the possible solutions, but it is necessary to look to the physical universe to find which one of these describes the process. Even the galaxies in our Local Group aren't expanding away from one another. So do the planets, moons, and stars, as well as the distances separating them. $^\dagger$ You would of course find more particles per cm 3 inside the Solar System than in the intergalactic medium, which would give you a hint, but space itself would be the same.Įven though the fabric of space is expanding throughout the universe, our solar system is not. But it doesn't! Space in sufficiently dense regions of the Universe is static. In order to be observed with zero redshift, space inside the Milky Way (or our Solar System) would - during the time that the photon was finishing the last part of its journey - have to contract by the same factor by which the Universe had expanded during all the time from the photon left its mother galaxy and until it approached the Milky Way. If suddenly the Universe froze in time, you would not be able to measure a difference between intergalactic space (which has followed the global expansion), and space inside our Solar System (which stopped expanding when the proto-Milky Way decoupled from the Hubble flow in the early Universe and started collapsing) $^\dagger$. There is no underlying coordinate system where you can check the absolute size of space. Remember, however, that you don't "see" the scale factor as you move around in space. I think this is the source of your (very fair) confusion. In terms of the scale factor $a$ of the metric, the redshift $z$ is given by However, there is also a general consensus that atoms, objects or systems held together by gravitational forces like our solar system do not take part in this expansion (see If the universe is expanding, does that mean atoms are getting bigger?). According to this theory, the fabric of space itself stretches during this expansion, hence increasing the wavelength of light (as qualitatively explained at A Model of the Universe). In this case, \(\Omega > 1\).It is generally accepted that the large scale redshift of galaxies (as given by the Hubble law) is due to the expansion of the universe. Matter terms dominate: If the Universe contains enough mass to counteract its expansion and there is no dark energy, it will eventually collapse.If there is no dark energy, the Universe will continue to expand, but increasingly slowly. Critical Universe: We have already discussed the case of a critical Universe, where the expansion and density terms are equal, \(\Omega = 1\), and space overall is not curved.One of the major efforts in cosmology over the past several decades has been to determine the value of \(\Omega\).įrom the Friedmann equation, we can see that the interplay of the expansion of the Universe, density, and curvature lead to several possibilities for the fate of the Universe, depending on which term in the equation dominates:
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