In between these matter-rich structures are enormous cosmic voids: regions with less matter than average, often spanning hundreds of millions or even billions of light-years in diameter. Note the fact that small-scale structure appears early on in all cases, while structure on larger scales does not arise until much later. The type and abundance of dark matter would deliver a vastly different Universe if we altered what our Universe possesses. The evolution of large-scale structure in the Universe, from an early, uniform state to the. Over millions, and even billions, of years, matter begins to clump and cluster together, forming stars, galaxies, galaxy groups and clusters, and even, on the largest scales, the great cosmic web. Even as the Universe expands, the initially overdense regions work to draw more and more of the surrounding matter into them, while the initially underdense regions do the opposite: preferentially giving up their matter to the more dense regions surrounding them. Matter gets less dense as the volume increases, while radiation not only dilutes but loses energy as it travels through the expanding Universe, as its wavelength - which defines its energy - gets stretched to a longer, redder state.Įven though the Universe came into existence with only tiny imperfections to its uniformity, with deviations of just ~0.003% from the mean value, on average, gravitation is relentless and cosmic timescales are incredibly long. Yes, the Universe is expanding the entire time, with the expansion rate gradually changing as the Universe’s density drops. The reason for this is simple: even though the Universe starts off incredibly uniform, it’s had 13.8 billion years for gravitation to do its thing.
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