- 3085 - UNIVERSE - explaining it s rate of expansion? Determining how rapidly the universe is expanding is key to understanding the fate of the Universe. But with more precise data this fate has become a mystery. Estimates based on measurements within our local universe don't agree with extrapolations from the era shortly after the Big Bang 13.8 billion years ago.
------------------- 3085 - UNIVERSE - explaining it s rate of expansion?
- A new estimate of the local expansion rate, the Hubble constant, or H0, reinforces this discrepancy.
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- Using a more precise technique for measuring cosmic distances employs the average stellar brightness within giant elliptical galaxies as a rung on the distance ladder. Astronomers calculate a rate to be 73.3 kilometers per second per megaparsec, give or take 2.5 km/sec/Mpc.
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- This result lies in the middle of three other good estimates, including the gold standard estimate from Type 1a supernovae. This means that for every mega parsec, 3.3 million light years, or 3 billion trillion kilometers, from Earth, the universe is expanding an extra 73.3 ±2.5 kilometers per second. The average from the three other techniques is 73.5 ±1.4 km/sec/Mpc.
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- Perplexingly, estimates of the local expansion rate based on measured fluctuations in the cosmic microwave background and, independently, fluctuations in the density of normal matter in the early universe (baryon acoustic oscillations), give a very different answer: 67.4 ±0.5 km/sec/Mpc. Note that this is 9% lower but with a measurement error 20% less.
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- Astronomers are understandably concerned about this mismatch, because the expansion rate is a critical parameter in understanding the physics and evolution of the universe and is key to understanding dark energy.
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- “Dark energy” is that unknown quantity that accelerates the rate of expansion of the universe and thus causes the Hubble constant to change more rapidly than expected with increasing distance from Earth. Dark energy comprises about two-thirds of the mass and energy in the universe, but is still a mystery. Remember mass is just another form of energy.
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- For the new estimate, astronomers measured fluctuations in the surface brightness of 63 giant elliptical galaxies to determine the distance and plotted distance against velocity for each to obtain H0.
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- The “surface brightness fluctuation” (SBF) technique is independent of other techniques and has the potential to provide more precise distance estimates than other methods within about 100 Mpc of Earth, or 330 million light years.
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- The 63 galaxies in the sample are at distances ranging from 15 to 99 Mpc, looking back in time a mere fraction of the age of the universe.
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- This astronomy is in the effort to understand the absolute scale of the universe, which then tells us about the physics. It started going back to James Cook's voyage to Tahiti in 1769 to measure a transit of Venus so that scientists could calculate the true size of the solar system.
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- The SBF method is more broadly applicable to the general population of evolved galaxies in the local universe, and certainly if we get enough galaxies with the James Webb Space Telescope, this method has the potential to give the best local measurement of the Hubble constant of expansion.
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- Astronomers over the years have laddered up to greater distances, starting with calculating the distance to objects close enough that they seem to move slightly, because of parallax, as the Earth orbits the sun.
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- Variable stars called “Cepheids” get you farther, because their brightness is linked to their period of variability, and Type Ia supernovae get you even farther, because they are extremely powerful explosions that, at their peak, shine as bright as a whole galaxy.
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- For both Cepheids and Type 1a supernovae, it's possible to figure out the absolute brightness from the way they change over time, and then the distance can be calculated from their apparent brightness as seen from Earth.
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- The best current estimate of H0 comes from distances determined by Type 1a supernova explosions in distant galaxies. Time delays caused by gravitational lensing of distant quasars and the brightness of water masers orbiting black holes, all give around the same number as supernovae.
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- The technique using surface brightness fluctuations is one of the newest and relies on the fact that giant elliptical galaxies are old and have a consistent population of old stars, red giant stars, that can be modeled to give an average infrared brightness across their surface.
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- The researchers obtained high-resolution infrared images of each galaxy with the Wide Field Camera 3 on the Hubble Space Telescope and determined how much each pixel in the image differed from the "average”. The smoother the fluctuations over the entire image, the farther the galaxy was, once corrections are made for blemishes like bright star-forming regions, which are excluded from the analysis.
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- The expansion rate came out close to that of the other local measurements. But astronomers are confounded by the glaring conflict with estimates from the early universe. A conflict that many astronomers say means that our current cosmological theories are wrong, or at least incomplete.
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- The extrapolations from the early universe are based on the simplest cosmological theory, lambda cold dark matter, or lambda CDM, which employs just a few parameters to describe the evolution of the universe.
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- One of the giants of the field, astronomer Wendy Freedman, recently published a study pegging the Hubble constant at 69.8 ±1.9 km/sec/Mpc, roiling the waters even further.
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- The latest result from Adam Riess, an astronomer who shared the 2011 Nobel Prize in Physics for discovering dark energy, reports 73.2 ±1.3 km/sec/Mpc.
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- The MASSIVE survey team used this method last year, 2020, to determine the distance to a giant elliptical galaxy, NGC 1453, in the southern sky constellation of Eridanus. Combining that distance, 166 million light years, with extensive spectroscopic data from the Gemini and McDonald telescopes to measure the velocities of the stars near the center of the galaxy, they concluded that NGC 1453 has a central black hole with a mass nearly 3 billion times that of the sun.
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- To determine H0 they calculated SBF distances to 43 of the galaxies in the MASSIVE survey, based on 45 to 90 minutes of HST observing time for each galaxy. The other 20 came from another survey that employed HST to image large galaxies, specifically ones in which Type Ia supernovae have been detected.
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- Most of the 63 galaxies are between 8 and 12 billion years old, which means that they contain a large population of old red stars, which are key to the SBF method and can also be used to improve the precision of distance calculations.
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- Astronomers employed both Cepheid variable stars and a technique that uses the brightest red giant stars in a galaxy, the tip of the red giant branch, or TRGB technique, to ladder up to galaxies at large distances. The TRGB technique takes account of the fact that the brightest red giants in galaxies have about the same absolute brightness.
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- If you take a look at any galaxy in the Universe that isn’t gravitationally bound to our own, we’ve already learned what’s going to happen to it in the future. Our Local Group, consisting of our Milky Way, Andromeda, and about 60 smaller galaxies, are the only ones bound to us.
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- If you considered any other galaxy as part of the bound structure that entire structure is receding from us, with its light systematically shifted towards longer wavelengths: a cosmic redshift. The farther away a galaxy is, on average, the greater the amount of its redshift, implying that the entireUniverse is expanding.
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- As time goes on, it will redshift by greater and greater amounts, implying that the Universe is not only expanding, but that it’s accelerating.
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- The inferred speed for any galaxy (that isn’t gravitationally bound to us) will rise over time, and all such galaxies will eventually become unreachable, even at the speed of light. And yet, if we were to measure the expansion rate of the Universe, what we call the Hubble constant, we’d find that it’s actually dropping over time, not rising.
Here’s how, in an accelerating Universe, that’s actually possible.
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- The first thing you have to realize is that in our theory of gravity, Einstein’s General Relativity, there’s a tremendously powerful relationship between the matter and energy in our Universe and the way that space and time behave. The presence, amount, and types of matter and energy present determine how space and time curve and evolve over time, and that curved spacetime tells matter and energy how to move.
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- In modern cosmology, a large-scale web of dark matter and normal matter permeates the Universe. On the scales of individual galaxies and smaller, the structures formed by matter are highly non-linear, with densities that depart from the average density by enormous amounts. On very large scales, however, the density of any region of space is very close to the average density: to about 99.99% accuracy.
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- A Universe filled with the same amount of stuff everywhere, from the earliest times (which we see imprinted in the Cosmic Microwave Background) to the present day (where we can count galaxies and quasars), seems to be exactly what we have. And if that’s the Universe in which you live.
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- What this spacetime tells us is remarkable. On one side of the equation, you get all the different forms of energy that can be present:
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------------------------------- normal matter,
------------------------------- antimatter,
------------------------------- dark matter,
------------------------------- neutrinos,
------------------------------- radiation (like photons),
------------------------------- dark energy,
------------------------------- spatial curvature,
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- And on the other side of the equation we have an expression that we quickly realized was how the fabric of space changed over time: either growing or shrinking. We could only tell which one was true by observing it.
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This equation tells us how the Universe evolves over time. Think about what it means: the rate at which the Universe either expands or contracts is directly related to the sum total of all the matter and energy, in all its different forms, present within it.
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- Before we had ever measured it, the widespread assumption was that the Universe was neither expanding nor contracting, but static. When Einstein realized that his equations predicted that a Universe full of stuff would be unstable against gravitational collapse, he threw in a cosmological constant to exactly balance out the force of gravity; the only way he could think of to prevent the Universe from imploding in a Big Crunch.
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- When the key observations of Hubble came in, the results were unmistakable: the Universe was indeed expanding, and completely inconsistent with a static solution.
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- An expanding Universe is one that was smaller in the past, and grows to occupy larger and larger volumes in the future. It’s one that was hotter in the past, since radiation is defined by the size of its wavelength, and as the Universe expands, this expansion stretches the wavelengths of any photons as they travel through intergalactic space, with the amount of stretching related to the amount of cooling.
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- It is one that was even more uniform in the past, as an almost-uniform Universe that gravitates will see those tiny initial over densities grow into the large-scale structure we observe today.
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- The big question is how the Universe’s expansion rate changes over time, and that’s dependent on the different forms of energy that are present within it. The volume of the Universe will continue to grow regardless of what’s in it, but the rate at which the Universe grows will change dependent on exactly what types of energy it’s filled with.
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- For various components of and contributors to the Universe’s energy density, and when they might dominate note that radiation is dominant over matter for roughly the first 9,000 years, then matter dominates, and finally, a cosmological constant emerges.
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- If we had a Universe that was 100% made of matter, with nothing else at all, it would expand at a rate that grew where if you doubled the age of the Universe, your size (in each of the three dimensions) would grow by 58%, while your volume would roughly quadruple.
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- If we had a Universe that was 100% made of radiation, again with nothing else at all, it would expand at a rate that grew you doubled the age of your Universe, your size would increase by 41% in each dimension, while the volume increases to about 2.8 times its original value.
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- And if you had a Universe that was filled with dark energy the Universe wouldn’t expand as a power law in time, but as an exponential. It would grow as e^Ht, where H is the expansion rate at any particular moment in time.
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- All three of these solutions are derivable from the Friedmann equations, and these solutions can be combined to represent a Universe with all three components, much like our own.
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- Why are these three cases so different from one another? The matter-filled Universe dilutes; its density drops as the volume expands, all while the mass (and hence the energy, since E = mc²) remains constant. As the energy density drops, so does the expansion rate.
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- The radiation-filled Universe dilutes faster; it’s density drops as the volume expands, while each individual photon also loses energy due to its cosmological redshift. The energy density drops faster for a radiation-filled Universe than a matter-filled one, and therefore so does the expansion rate.
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- But a Universe filled with dark energy, a cosmological constant, doesn’t dilute. The energy density remains constant: the definition of a cosmological constant. As the volume of the Universe expands, the total amount of energy goes up, keeping the expansion rate constant.
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- While matter (both normal and dark) and radiation become less dense as the Universe expands owing to its increasing volume, dark energy, and also the field energy during inflation, is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant.
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- If you were to then imagine that, in each of these Universes, you were located at the same point, and there were one other galaxy in the Universe (corresponding to a different point), you could watch it recede away from you over time. You could measure how its distance was changing with time, and you could measure how its redshift (which corresponds to its recession speed) changed with time.
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- In the matter-filled Universe, the other galaxy would get farther and farther away from you as time went on, but it moves away from you more slowly in the process. Gravity works to counteract the expansion, failing to stop it but succeeding in slowing it down. In a matter-only Universe, the expansion rate continues to drop, eventually approaching zero.
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- In the radiation-filled Universe, the other galaxy still gets farther and farther away as time goes on, but the galaxy not only moves away more slowly as time goes on, it slows down faster than in the matter-only case. The expansion rate still asymptotes to zero, but the distant galaxy remains closer and moves away more slowly than in the matter-filled version.
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- But in the dark energy-filled Universe, the other galaxy gets farther away and does so at an increasingly faster speed. When it’s double the initial distance away, it now appears to be receding at double the speed. At 10 times the distance, it’s 10 times the speed. Even though the expansion rate is a constant, any individual galaxy speeds up as it recedes from us over time.
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- The expansion rate, today, is 70 km/s/Mpc. The expansion rate is a speed (70 km/s) that accumulates with cosmic distance (for each Mpc, or megaparsec, which corresponds to 3.26 million light-years). If something’s 10 Mpc away, it recedes at 700 km/s; if it’s 1,000 Mpc away, it recedes at 70,000 km/s.
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- In a matter-filled or radiation-filled Universe, the expansion rate itself drops with time, so even as a galaxy gets more distant, the expansion rate slows down by a greater percentage than its distance goes up. But in a dark energy-filled Universe, the expansion rate is constant, so as a galaxy gets more distant, it moves away faster and faster.
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- The largest contributors to our Universe’s energy today are matter (at 32%) and dark energy (at 68%). The matter part continues to dilute, while the dark energy part remains constant. Since both contribute, the expansion rate continues to drop, and will eventually asymptote to a value of 45–50 km/s/Mpc.
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- However, a distant galaxy still speeds up as it moves away from us, something that’s been going on for the past 6 billion years in our 13.8 billion year history. The expansion rate is dropping, but the speeds of distant galaxies are still increasing, or accelerating.
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- The big key to understanding this is as the Universe expands, we can measure two different things. We can measure the expansion rate, which tells us, for every megaparsec a galaxy is away from us, how fast it recedes. This expansion rate, a speed-per-unit-distance, changes over time, dependent on the amount of energy present within a given volume of the Universe.
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- As the Universe expands, the amount of dark energy in a given volume stays the same, but the matter and energy densities go down, and therefore so does the expansion rate.
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- You can also measure a distant galaxy’s recession speed, and in a Universe dominated by dark energy, that speed will increase over time: an acceleration. The expansion rate drops, asymptoticly to a constant (but positive) value, while the expansion speed increases, accelerating into the oblivion of expanding space.
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- Both of these things are simultaneously true: the Universe is accelerating and the expansion rate is very slowly dropping. Now you finally understand what is happening in your Universe. Just try and explain it to your kids?
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March 10, 2021 UNIVERSE - explaining it s rate of expansion? 3085
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