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New instruments like DESI will help take the next step in resolving the mystery. It will measure tens of millions of galaxy redshifts, spanning a huge volume of the universe up to ten billion light years from Earth. Such an amazing, detailed map should be able to answer a few key questions about dark energy and the creation of the large scale structures in the universe.
For example, it should be able to tell us if dark energy is just a cosmological constant.
This Is How Astronomers Will Finally Measure The Universe’s Expansion Directly
To do this it will measure the ratio of pressure that dark energy puts on the universe to the energy per unit volume. If dark energy is a cosmological constant, this ratio should be constant in both cosmic time and location. For other explanations, however, this ratio would vary. Any indication that it is not a constant would be revolutionary and spark intense theoretical work.
Or the opposite — and again that would spark a revolution in theoretical physics. Another important theory that will be tested with DESI is Inflation, which predicts that tiny random quantum fluctuations of energy density in the primordial universe were exponentially expanded during a short period of intense growth to become the seeds of the large scale structures we see today. New satellite missions like Euclid , and massive ground based observatories like the Large Synoptic Survey Telescope , will also offer insights. Together, these will provide hundreds of millions of redshifts across the whole sky leading to an unimaginable map of our cosmos.
It seems a long time ago now when I wrote my PhD thesis based on just galaxy redshifts. In astronomy, the objects we see are often so far away and so grand in scale that, on human timescales, we have no way to measure how they change in real-time.
If the fabric of space is like a ball of dough, and the individual galaxies within the Universe are like raisins, then the expanding Universe is like the dough when it leavens. The raisins galaxies all appear to move away from one another, with more distant raisins galaxies appearing to recede more quickly. But this observation is primarily due to the fact that the dough Universe is expanding.
The fact that a whole slew of disparate data sets are all consistent with not only one another but with an expanding, evenly filled Universe in the context of relativity, that gives us the confidence we have in our model of the Universe. But when we move from having meter class telescopes to meter class telescopes, with approximately:. This represents an improvement of a factor of to over existing telescopes, and means that if we wait just a decade or perhaps a decade-and-a-half once the ELT comes online at full power, we should be able to measure the expansion of the Universe directly.
This includes:. By at the latest, we should be able to directly confirm the expansion of the Universe, putting our understanding of the cosmos to the ultimate test. Whether we discover something new or not is for nature to decide; we have no control over that. Sign in. Our universe was different in the past and will be different in the future. And that difference is due to expansion. Over time, galaxies on average get farther away from us and farther away from each other.
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We live in an expanding universe. But exactly how fast is it expanding today? Astronomers are struggling with this answer because different techniques are giving different results, and the resolution to this problem may come in the form of brand-new physics. Or maybe we're just getting something wrong.
Hard to tell. We understand the expansion of the universe through Einstein's theory of general relativity.
This is also how we understand all gravitational interactions in said universe. General relativity connects the amount and kinds of stuff you have in space with distortions of that space, giving rise to bends and folds and features.
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Then that geometry of space-time tells all the stuff how to move. Voila: gravity.
The universe is by definition made of stuff, and therefore is subject to general relativity. The amount and kinds of stuff at grand scales tell space-time how to bend at those same grand scales, and that bending of space-time tells all the matter how to move. So the ingredients of the universe affect the expansion rate. And in different points in cosmic history, different components have been in charge; the universe is made of many things like normal matter, radiation, dark matter, and dark energy.
Over the course of the past The equations of general relativity give us the glue to go from the ingredients at one moment of cosmic time to the expansion rate at that time or vice versa, to go from the expansion rate at that time to the ingredients.
How Big Is the Universe?
This is the magic of equations: they go in both directions of the equal sign. This means that cosmologists can either go out to try to measure the expansion rate directly, or try to measure the ingredients directly and calculate the expansion rate. One of our best tools for measuring the ingredients of the universe is the cosmic microwave background.
This is the leftover radiation from when the universe was just , years old. It's a veritable baby portrait of the entire cosmos.
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That light was released long ago, and it's been sailing through the void of space until it hit our detectors like Europe's Planck satellite, which very accurately measured the cosmic microwave background a few years ago. From that portrait we are able to accurately measure the ingredients in the universe.
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How much normal matter, how much dark matter , how much neutrinos, how much radiation and so on.
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