- 3641 - GALAXIES - explode to measure distance? Gamma-ray Bursts are explosions of stars inside galaxies. These explosions can help astronomers measure vast distances across the Universe. Now that the James Webb Space Telescope is operational, astronomers can study some of the most faint and distant galaxies ever seen.
--------------------- 3641 - GALAXIES - explode to measure distance?
- Our new James Webb telescope has already captured the image of a galaxy from when the universe was just 300 million years old. That is how old the Universe was, but, how do we know how far away it was?
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- How do you measure the distance of these farthest galaxy?
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- Astronomer rely on its observed redshift. Since the universe is expanding, the more distant the galaxy, the more redshifted its light. The wavelength of light gets stretched out to lower frequencies as it travels through expanding space. It can start out at Gamma Ray frequencies and by the time it gets to us it is stretched out to Microwave frequencies.
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- To calculate the galaxy’s distance, astronomers plug this redshift into a formula derived from the standard cosmological model. By observing everything from variable stars to distant supernovae, we know the relationship between redshift and distance really well. Do the math, and get the distance.
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- Of course, the most distant galaxies are farther away than the “calibrating observations” of the standard model. We can extrapolate the standard model for these galaxy distances, but that assumes the cosmic acceleration wasn’t radically different way back then.
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- There’s no reason to assume our assumption is wrong, but it would be nice to get distance measurements at the limits of Webb’s galaxies. Unfortunately, the type of supernovae we use for these distance measurements aren’t bright enough to be seen at that distance.
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- But, a new study finds we might be able to use gamma ray bursts instead of supernovae.
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- Gamma-ray bursts, or GRBs, are short-lived explosions of gamma radiation. They are caused by hypernova explosions of giant stars, and they are extremely powerful. A single GRB releases more energy than the Sun will emit in its entire 10-billion-year lifetime. But the light curves of GRBs are complex, making it difficult to find a common pattern.
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- In this study astronomers didn’t look at the variation of gamma-ray light but instead looked at optical light. They looked at the optical light curves of 500 known GRBs and found nearly 180 with a common pattern. Many of these GRBs are close enough that we know their distances, so the team was able to use their observed brightnesses to calculate the actual brightnesses.
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- This means that if a really distant GRB of this common type is detected, astronomers can use its observed brightness to calculate its distance, thus extending our cosmic distance ladder.
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- To understand where our Universe came from and where it’s going, we need to measure how it’s expanding. If everything is moving away from everything else, we can extrapolate in either direction to figure out both our past and our future. Go backwards, and things will get denser, hotter, and less clumpy.
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- If you know the expansion rate now and what’s in your Universe, we can go all the way back to the Big Bang. Similarly, if we know the expansion rate now and how it’s changing over time, we can go all the way forward to the heat death of the Universe.
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- 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. How do we even get those rates?
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- “The Cosmic Microwave Background (CMB) is a very important part of the Big Bang model. How do they calculate H0, the rate of expansion, from the CMB?
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- The Universe starts off very hot, dense, and uniform. As it ages, it expands; as it expands, it gets:
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--------------------- Cooler (because the radiation in it gets stretched in wavelength, shifting it towards lower energies and temperatures),
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--------------------- Less dense (because the number of particles in it stays constant, but the volume increases),
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-------------------- Clumpier (because gravity pulls more matter into the denser regions, while preferentially stealing matter away from the less-dense regions).
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- As all of these things happen, the expansion rate also changes, getting smaller with time. There are many different ways to go about measuring the expansion rate of the Universe, but they all fall into two categories: what I call the “distance ladder” method and the “early relic” method.
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- The “distance ladder method” is easier to understand. 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. Do this for enough objects at a variety of distances and you’ll reveal how quickly the Universe is expanding, with very small errors and uncertainties.
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- You can measure individual stars directly, determining their distance simply by measuring them throughout the year. 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.
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- Once you know how far away those types of stars are, Cepheids, RR Lyrae, certain types of giant stars, you can look for them in distant galaxies. Because you know how these stars work, you can determine their distances, and therefore the distances to those galaxies.
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- Then, you can measure properties of those galaxies or objects within those galaxies:
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----------------- Rotation properties, velocity dispersions, surface brightness fluctuations, individual events like type 1a supernovae, etc.
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- 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.
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- The “early relic methods” are more complicated , but not necessarily more complicated as a concept. 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 early time. We then measure a signal that’s observable today that’s affected in a specific way by that early imprint.
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- 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. 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.
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- The so-called “Key Project” from the Hubble Space Telescope, named because it’s goal was to measure the Hubble constant of space expansion, returned their results:
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------------------ The Universe was expanding at 72 km/s/Mpc, with an uncertainty of about 10%. This translates to:
------------------ 49,300 miles per hour for every million lightyears in distance.
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- Within the distance ladder class, all the measurements appear to converge on a value that’s 73-74 km/s/Mpc, but within the early relic class, all the measurements appear to converge on a value that’s 67-68 km/s/Mpc.
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- The uncertainties on these values are about 1-2% each, but they differ by about 9% from one another. Unless something is fundamentally wrong with one of these classes of measurement or there’s some type of physics we aren’t accounting for, this mystery isn’t really going anywhere anytime soon.
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- To understand where that CMB value and what the CMB is telling us; The early Universe was hot and dense: so hot and so dense that, at some point long ago, it wasn’t possible to form neutral atoms. Anytime a proton or any atomic nucleus encountered an electron, the electron would attempt to bind to it, cascading down the various energy levels and emitting photons.
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- But if your Universe is too hot, there are going to be photons that are energetic enough to kick those electrons right back off again. It’s only once the Universe has had enough time to expand and cool, and all the photons in it have cooled (on average) to below a certain temperature, that you can form those neutral atoms. At that point, when the neutral atoms form, those photons stop bouncing off of the free electrons, because there are no more free electrons; they’ve all been bound up in neutral atoms, and that light simply travels in a straight line at the speed of light until it hits something.
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- Most of that light hasn’t hit anything, because space is mostly empty. When we look out at the sky today, we see that leftover light, although we don’t see it exactly as it was when it was released by those neutral atoms. Instead, we see it as it is today, after journeying through the expanding Universe for some 13.8 billion years.
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- It was about 3,000 Kelvin in temperature when the Universe first became neutral; it’s cooled down to 2.7255 K today. Instead of peaking in the visible part of the spectrum or even the infrared part, the light has shifted so severely it now appears in the microwave portion of the spectrum.
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- That 2.7255 K is the same everywhere: in all directions that we look. We’re moving through the Universe relative to this background of light, causing the direction we’re moving in to appear hotter and the direction we’re moving away from to appear colder. When we subtract that effect out, we discover that down at about the 0.003% level, temperature differences of only tens or hundreds of micro-degrees, there are temperature fluctuations: places that are ever so slightly hotter or colder than average.
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- This is the crux of the big question: how do we get the expansion rate from these measurements of temperature and temperature fluctuations?
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- If you start with a Universe with a known set of ingredients at the earliest times, at the start of the hot Big Bang, and, you know the equations that govern your Universe, you can calculate how your Universe will evolve from that early stage until 380,000 years have passed: the time that the Universe has cooled to 3,000 K and will release the CMB.
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- Every different set of ingredients that you put in will have its own unique CMB that it produces. If you calculate how a Universe behaves with normal matter and radiation only, you only get about half the “wiggle” features that you’d get in a Universe with dark matter, too. If you add too much normal matter, the peaks get too high.
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- If you add in spatial curvature, the size scales of the fluctuations change, getting smaller or larger (on average) depending on whether the curvature is positive or negative. And so on.
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- In doing this analysis there are certain parameters that you can all vary together. A little more dark and normal matter, a little more dark energy, a lot more curvature, a slower expansion rate, etc. that will all yield the same patterns of fluctuations. In physics, we call this a “degeneracy“.
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- The temperature spectrum of the CMB is inherently degenerate: there are multiple possible cosmologies that can reproduce the patterns we see. But there are other components to the CMB as well, besides the temperature spectrum.
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- There’s “polarization“. There’s a temperature-polarization cross-spectrum. There are different initial sets of fluctuations that the Universe could start off with in different models of inflation.
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- The range of possible cosmologies that can work to fit the CMB are fairly narrow. The best-fit value comes in at 67-68 km/s/Mpc for the expansion rate, corresponding to a Universe with about 32% matter (5% normal matter and 27% dark matter) and 68% dark energy.
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- If you try to move the expansion rate lower, you need more normal-and-dark matter, less dark energy, and a slight amount of positive spatial curvature. If you try to move the expansion rate higher, you need less total matter and more dark energy, and possibly a little bit of negative spatial curvature. There’s very little actual wiggle-room, especially when you start considering other independent constraints.
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- The abundances of the light elements, for instance, tell us precisely how much normal matter exists. The measurements of galaxy clusters and large-scale structure tell us how much total matter, normal and dark combined, exists.
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- And all the different constraints, together, tell us the age of the Universe: 13.8 billion years, with an uncertainty of only 1%. The CMB is not just one data set, but many, and they all point towards the same picture. It’s all self-consistent, but it doesn’t paint the same picture that the cosmic distance ladder does. This will remain one of the biggest conundrums in modern cosmology.
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- Most short gamma-ray bursts observed so far have happened in bright, nearby galaxies. But 43 of the ones detected by NASA’s Swift Observatory seemed to have come from the middle of nowhere, distant reaches of intergalactic space with no galaxy in sight.
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- Short gamma-ray bursts are short bursts of gamma rays. But they leave behind an afterglow of longer-wavelength radiation like X-rays. When Swift detects a gamma-ray burst, it uses the X-ray afterglow to pinpoint where in space the explosion came from.
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- The gamma-ray bursts coming from those galaxies happened 8 to 10 billion years ago, when the universe was much younger.
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- Collisions between neutron stars are powerful enough to forge entirely new elements, including heavy metals like gold, platinum, and thorium. If neutron stars were colliding to produce short gamma-ray bursts 8 to 10 billion years ago, then they were also seeding the young universe with heavy elements much sooner than anyone thought.
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- This pushes the timescale back on when the universe received and became seeded with the heaviest elements in the periodic table.
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August 2, 2022 GALAXIES - explode to measure distance? 3641
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