- 3518 - UNIVERSE - expanding into what? To understand where our Universe came from and where it’s going, we need to measure how fast it is expanding. If everything is moving away from everything else, we can extrapolate in either direction to figure out both our past and our future.
--------------------- 3518 - UNIVERSE - expanding into what?
- Go backwards, and things get denser, hotter, and less clumpy. If you know the expansion rate now and what’s inside your Universe, you can go all the way back to the Big Bang with this simple math.
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- If you know the expansion rate now and how it’s changing over time, you can also go all the way forward to the “heat death” of the Universe. 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. What?
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- The Cosmic Microwave Background (CMB) is another very important part of the Big Bang model.
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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: the “distance ladder” method and the “early relic” method.
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- The “distance ladder method” is easier to understand. 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.
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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, how bright they are, you can determine their distances, and therefore the distances to those galaxies. Physics knows how much a light source fades with distance.
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- 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. As long as you can measure the properties 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 in detail, 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 some early time.
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- We can measure an imprint todayand we can learn how the Universe expanded from the moment that early relic was imprinted to right now.
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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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- No matter how we measure the expansion rate of the Universe, we’d get precisely the same answer. In the late 1990s/early 2000s, we thought we had finally pinned it down.
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- The “Key Project” from the Hubble Space Telescope, named because it’s goal was to measure the Hubble constant, returned their main results: the Universe was expanding at 72 km/s/Mpc, with an uncertainty of about 10%. Since that 2001 release, these various methods have beaten those uncertainties down further.
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---------------------- 72 kilometers per second per mega parsec
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----------------------- is: 49,300 miles per hour per million lightyears distance
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- There’s a controversy in cosmology today because 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.
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- If we want to understand where that CMB value comes from, you have to understand what the CMB is and what it’s 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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- 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.
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- 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 light simply does what it does: travel in a straight line at the speed of light until it hits something.
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- Of course, 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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- The light 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 because the wavelengths have been stretched.
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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.
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- 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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- 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 Cosmic Microwave background energy.
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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.
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- If you add too much normal matter, the peaks get too high. If you add in spatial curvature, the size scales of the fluctuations change, getting smaller or larger depending on whether the curvature is positive or negative.
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- What’s fascinating about doing this analysis is that 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. will all yield the same patterns of fluctuations.
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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. Similarly, 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 when you start considering other independent constraints.
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- The abundances of the light elements 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. And all the different constraints, together, tell us the age of the Universe: 13.8 billion years, with an uncertainty of only 1%.
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- 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. Until we figure out why, this will remain one of the biggest conundrums in modern cosmology.
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- Every new estimate of the local expansion rate, the Hubble constant, or H0 (H-naught) ,reinforces that discrepancy. Using a relatively new 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:
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--------------------- 73.3 kilometers per second per megaparsec, +/- 2.5 km/sec/Mpc
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- 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.
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- 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:
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-------------------- 67.4 ±0.5 km/sec/Mpc.
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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 which 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.
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- Dark energy comprises about two-thirds of the mass and energy in the universe, but is still a mystery.
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- For another 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. 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 whole story of astronomy is the effort to understand the absolute scale of the universe. 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 Hubble constant has been a bone of contention for decades, ever since Edwin Hubble first measured the local expansion rate and came up with an answer seven times too big, implying that the universe was actually younger than its oldest stars. The problem, then and now, lies in pinning down the location of objects in space that give few clues about how far away they are.
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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 Ia 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 Ia supernova explosions in distant galaxies, though newer methods like time delays caused by gravitational lensing of distant quasars and the brightness of water masers orbiting black holes all give around the same number.
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- The technique using surface brightness fluctuations relies on the fact that giant elliptical galaxies are old and have a consistent population of old stars, mostly red giant stars, that can be modeled to give an average infrared brightness across their surface.
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- High-resolution infrared images of each galaxy with the Wide Field Camera 3 on the Hubble Space Telescope determined how much each pixel in the image differed from the “average” , the smoother the fluctuations over the entire image, the farther the galaxy, once corrections are made for blemishes like bright star-forming regions.
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- The extrapolations from the early universe are based on the simplest cosmological theory, the “lambda cold dark matter“, or CDM, which employs just a few parameters to describe the evolution of the universe.
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- Astronomer Wendy Freedman 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, reports 73.2 ±1.3 km/sec/Mpc.
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- To determine H0 astronomers 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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- Both Cepheid variable stars and a technique that uses the brightest red giant stars in a galaxy, referred to as the tip of the red giant branch, or TRGB technique were used 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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- The James Webb telescope has the potential to really decrease the error bars for SBF.
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- Even if you took away every single quantum of energy, somehow removing it from the Universe entirely, you wouldn’t be left with an empty Universe. No matter how much you take out of it, the Universe will always generate new forms of energy.
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- How is this possible? If we were to remove all the quanta of energy from our Universe, leaving behind only empty space, we would immediately expect that the Universe would be at absolute zero: with no energetic particles anywhere to be found. Yet that’s not the case at all.
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- No matter how “empty” we artificially make the expanding Universe, the fact that it’s expanding would still spontaneously and unavoidably generate radiation. Even arbitrarily far into the future, or all the way back before the hot Big Bang, the Universe would never truly be empty.
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- in our Universe today, it’s very clear that space is anything but empty. In every direction we look, we see:
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------------------------- stars, gas, dust, other galaxies, galaxy clusters, quasars,
high-energy cosmic particles (known as cosmic rays), radiation, both from starlight left over from the Big Bang itself.
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- If we had better “eyes,” we’d see gravitational waves from every mass that’s accelerating through a changing gravitational field. We’d “see” whatever is responsible for dark matter, rather than simply its gravitational effects. We’d see blackholes, both active and quiescent, rather than simply the ones that are emitting the greatest amounts of radiation.
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- The spacetime fabric of our Universe is in the process of expanding. If you put any two well-separated “points” down in your spacetime, you’ll find that the:
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--------------------- Proper distance (as measured by an observer at one of the points) between those points,
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--------------------- The light-travel time between those points,
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--------------------- The wavelength of the light that travels from one point to the other,
will all increase over time.
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- The Universe is not just expanding, but also cooling concurrently as a result of the expansion.
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- As light shifts to longer wavelengths, it also shifts towards lower energies and cooler temperatures; the Universe was hotter in the past and will be even colder in the future. Through it all, the objects with mass and/or energy in the Universe gravitate, clumping and clustering together to form a great cosmic web.
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- If you could somehow eliminate it all, all the matter, all the radiation, every single quanta of energy, what would be left?
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- You’d just have empty space itself, still expanding, still with the laws of physics intact, and still with the inability to escape the quantum fields that permeate the Universe. This is the closest you can get, physically, to a true state of “nothingness,” and yet it still has physical rules it must obey.
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- What we perceive as “dark energy” today would still exist in this “Universe of nothing” that we’re imagining. In theory, you can take every quantum field in the Universe and put it into its lowest-energy configuration. If you do this, you’d reach what we call the “zero-point energy” of space, which means that no more energy can ever be taken out of it and put to use performing some type of mechanical work.
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- In a Universe with dark energy, a cosmological constant, or the zero-point energy of quantum fields, there’s no reason to infer that the zero-point energy would actually be zero.
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- In our Universe it’s observed to have a finite but positive value: a value that corresponds to an energy density of about 1 GeV (about the rest mass energy of a proton) per cubic meter of space.
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- This is a tremendously small amount of energy. If you took the energy inherent in a single human body, largely from the mass of your atoms, and spread it out to have the same energy density as the zero-point energy of space, you’d find that you occupied as much space as a sphere that was roughly the volume of the Sun!
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- In the very far future the Universe will behave as though this zero-point energy is the only thing left within it. The stars will all burn out; the corpses of these stars will radiate all their heat away and cool to absolute zero; the stellar remnants will gravitationally interact, ejecting the majority of objects into intergalactic space, while the few remaining black holes grow to enormous sizes. Eventually, even they will decay away through Hawking radiation, and that’s where the story really gets interesting.
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- The idea that blackholes decay might be justifiably remembered as Stephen Hawking’s most important contribution to science, but it holds some important lessons that go well beyond blackholes. Blackholes have what’s called an “event horizon“: a region that once anything from our Universe crosses over this imaginary surface, we can no longer receive signals from it.
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- We think of blackholes as the volume inside the event horizon: the region from which nothing, not even light, can escape. But if you give it enough time, these blackholes will evaporate completely.
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- Blackholes evaporate because they radiate energy, and that energy gets drawn from the mass of the blackhole, converting mass to energy via Einstein's E = mc². Close to the event horizon, space is more severely curved; farther from the event horizon, it's less curved. This difference in curvature corresponds to a disagreement as to what the zero-point energy of space is.
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- Someone close to the event horizon will see that their “empty space” is different from the “empty space” of someone farther away, and that's a problem because quantum fields are continuous and occupy all of space.
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- The key thing to realize is that if you're at any location outside of the event horizon, there's at least one possible path that light could take to travel to any other location that's also outside of the event horizon. The difference in the zero-point energy of space between those two locations tells us, as first derived in Hawking’s 1974 paper, that radiation will be emitted from the region around the blackhole, where space is curved the most strongly.
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- The presence of the blackhole’s event horizon is important here, while the spectrum of the radiation is a perfect blackbody and its temperature is set by the blackhole's mass: lower masses are hotter and heavier masses are colder.
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- The expanding Universe has a “cosmic horizon“. In an expanding Universe you have to be located close enough to one another so that the expansion of spacetime between those two points doesn’t prevent emitted light from ever arriving.
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- In our present-day Universe, that corresponds to a distance that’s approximately 18 billion light-years away. If we emitted light right now, any observer within 18 billion light-years of us could eventually receive it; anyone farther away never would, owing to the Universe’s ongoing expansion.
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- We can see farther away than that because many sources of light were emitted long ago. The earliest light that’s arriving right now, 13.8 billion years after the Big Bang, is from a point that’s presently about 46 billion light-years away.
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- If we were willing to wait an eternity, we’d eventually receive light from objects that are presently as far away as 61 billion light-years; that’s the ultimate limit.
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- From any observer’s point of view, there exists this cosmological horizon: a point beyond which communication is impossible, since the expansion of space will prevent observers at these locations from exchanging signals beyond a certain point in time.
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- And just like the existence of a blackhole’s event horizon results in the creation of Hawking radiation, the existence of a cosmological horizon must also create radiation. In this case, the prediction is that the Universe will be filled with extraordinarily low-energy radiation whose wavelength is, on average, of a size comparable to the cosmic horizon. That translates into a temperature of 10-30 Kelvin: thirty orders of magnitude weaker than the current Cosmic Microwave Background.
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- As the Universe continues to expand and cool, there will come a time in the far-distant future where this radiation becomes dominant over all the other forms of matter and radiation within the Universe; only dark energy will remain a more dominant component.
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- Before the hot Big Bang occurred, our Universe was expanding at an enormous and relentless rate. Instead of being dominated by matter and radiation, our cosmos was dominated by the field energy of inflation: just like today’s dark energy, but many orders of magnitude greater in strength and expansion speed.
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- Although inflation stretches the Universe flat and expands any pre-existing particles away from one another, this doesn’t necessarily mean the temperature approaches and asymptotes to absolute zero in short order. Instead, this expansion-induced radiation, as a consequence of the cosmological horizon, should actually peak in infrared wavelengths, corresponding to a temperature of about 100 K, or hot enough to boil liquid nitrogen.
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- What this means is that if you ever wanted to cool the Universe down to absolute zero, you’d need to stop its expansion entirely. So long as the fabric of space itself has a non-zero amount of energy intrinsic to it, it will expand. So long as the Universe expands relentlessly, there will be regions separated by a distance so great that light, no matter how long we wait, will not be able to reach one such region from the other. And as long as certain regions are unreachable, we will have a cosmological horizon in our Universe, and a bath of thermal, low-energy radiation that can never be removed.
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- Insisting that the laws of physics remain valid is enough to do away with the idea of a truly empty Universe. So long as energy exists within it — even the zero-point energy of the quantum vacuum is sufficient — there will always be some form of radiation that can never be removed. The Universe has never been completely empty, and so long as dark energy doesn’t decay entirely away, it never will be.
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March 28, 2022 UNIVERSE - expanding into what? 3518
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