![]() ![]() This leads, over time, to the large-scale structure in the Universe today, as well as the fluctuations in temperature observed in the CMB. inflation ends, they become density fluctuations. The quantum fluctuations that occur during inflation get stretched across the Universe, and when. They show up in the large-scale clustering of galaxies we see in the late-time Universe, and they also show up in the leftover glow from the Big Bang: the Cosmic Microwave Background, or the CMB. The two most famous “early relic” methods both come from the same source: those initially overdense and underdense regions that provided the seeds for the growth of large-scale structure in the Universe. When we measure that imprint today, we can learn how the Universe expanded from the moment that early relic was imprinted to right now, when we measure it. What’s changed? The Universe has expanded from the Big Bang to the present day. We then measure a signal that’s observable today that’s affected in a specific way by that early imprint. Instead of starting here on Earth and working our way out, deeper and deeper into the distant Universe, we start way back at the Big Bang, and calculate some initial imprint at some stupendously early time. The early relic methods, as a group, are more complicated in detail, but not necessarily more complicated as a concept. However, the fluctuations in the CMB, the formation and correlations between large-scale structure, and modern observations of gravitational lensing all point towards the same picture. matter and dark energy are required, and that we don't know the origin of any of these mysteries. As long as you can measure the properties you’re seeking, you’ll be able to build a cosmic distance ladder, determining how the Universe has expanded between the time the light was emitted from your distant objects and when it arrived at your eyes.Ī detailed look at the Universe reveals that it's made of matter and not antimatter, that dark. Then, you can measure properties of those galaxies or objects within those galaxies: rotation properties, velocity dispersions, surface brightness fluctuations, individual events like type Ia supernovae, etc. Because you know how these stars work, you can determine their distances, and therefore the distances to those galaxies. you can look for them in distant galaxies. Once you know how far away those types of stars are - Cepheids, RR Lyrae, certain types of giant stars, etc. As the Earth moves around the Sun, that tiny change in distance is enough to reveal how much the stars shift by, the same way your thumb shifts relative to the background if you close one eye and then switch eyes. You can measure individual stars directly, determining their distance simply by measuring them throughout the year. Do this for enough objects at a variety of distances - including large enough distances - and you’ll reveal how quickly the Universe is expanding, with very small errors and uncertainties.Īt this point, there are many different ways of doing this. All you’re going to do is measure objects that you understand, determining both their distance from you and how much the light from them gets shifted by the expansion of the Universe. The distance ladder method is easier to understand. Each “step” carries along its own uncertainties, but with many independent methods, it's impossible for any one rung, like parallax or Cepheids or supernovae, to cause the entirety of the discrepancy we find. The construction of the cosmic distance ladder involves going from our Solar System to the stars to. ![]() How do we even get those rates? That’s what Lindsay Forbes (no relation) wants to know, asking: But one of cosmology’s biggest puzzles is that we have two completely different methods for measuring the Universe’s expansion rate, and they don’t agree. Similarly, if you know the expansion rate now and how it’s changing over time, you can go all the way forward to the heat death of the Universe. If you know the expansion rate now and what’s in your Universe, you can go all the way back to the Big Bang. ![]() Go backwards, and things will get denser, hotter, and less clumpy. If everything is moving away from everything else, we can extrapolate in either direction to figure out both our past and our future. If you want to understand where our Universe came from and where it’s going, you need to measure how it’s expanding. Siegel / Damien George / / Planck Collaboration underlying these maps encodes a tremendous amount of information about the early Universe, including what it's made of and how quickly it's expanding. The hot and cold spots from the hemispheres of the sky, as they appear in the CMB. ![]()
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