- 3099 - UNIVERSE - why is it expanding? Determining how rapidly the universe is expanding is key to understanding our cosmic fate, but with more precise data has come 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. Which is the correct picture?
------------------- 3099 - UNIVERSE - why is it expanding?
- This newest estimate of the local expansion rate, the Hubble constant, or H0, reinforces that discrepancy.
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- Using a relatively new technique for measuring cosmic distances, employing the average stellar brightness within giant elliptical galaxies as a rung on the distance ladder, astronomers calculate an expansion rate:
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-------------------- Ho = 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, from Earth, the universe is expanding an extra 73.3 kilometers per second. The average from the three other techniques is 73.5 ±1.4 km/sec/Mpc.
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- Calculations 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 even a different answer:
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------------------- Ho = 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 that too is still a mystery.
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- For this new estimate of expansion rate, 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 Ho.
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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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- The whole story of astronomy is the effort to understand the absolute scale of the universe, which then tells us about the physics.
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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 1a 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 Ho comes from distances determined by Type 1a supernova explosions in distant galaxies, though newer methods including time delays caused by gravitational lensing of distant quasars and the brightness of water masers orbiting blackholes all three give around the same number.
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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, mostly 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, once corrections are made for blemishes like bright star-forming regions, which were excluded from the analysis.
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- Astronomer Wendy Freedman, published a study pegging the Hubble constant at
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--------------------------- 69.8 ±1.9 km/sec/Mpc.
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- Astronomer Adam Riess, shared the 2011 Nobel Prize in Physics for discovering dark energy, reports:
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-------------------------- 73.2 ±1.3 km/sec/Mpc.
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- When it comes to the physical Universe, the notion of “nothing” may truly be possible only in theory, not in practice. As we see the Universe expanding today, it appears full of stuff: matter, radiation, antimatter, neutrinos, and even dark matter and dark energy.
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- Even if you took away every single quantum of mass / 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? It’s like the Universe itself doesn’t understand our idea of “nothing”. 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.
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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. Why?
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------------------------------- In the universe today 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 and left over from the Big Bang .
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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. And 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 fabric of our Universe, spacetime, is in the process of expanding:
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---------------------- the 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, and 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. 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.
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- And, at the same time 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 would 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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- This means, “dark energy” 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.
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- 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, in fact, 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, 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 blackholes grow to enormous sizes.
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- Eventually, even blackholes will decay away through “Hawking radiation“.
- This 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.
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- 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. Typically, 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, photons will escape and these black holes will evaporate completely.
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- Why do these 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.
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- 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, at least as we understand them, 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 that radiation will be emitted from the region around the blackhole, where space is curved the most strongly.
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- 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 doesn’t have an event horizon, because it isn’t a blackhole. However, it does have something that’s analogous: a cosmic horizon. If you’re located anywhere in spacetime and you consider an observer at another location in spacetime, you’d immediately think, there must be at least one possible path light could take that connects me to this other observer.
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- But in an expanding Universe, that’s not necessarily true. 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. 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“. It is 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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- 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.
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- That all translates into a temperature of 10-30 Kelvin: thirty orders of magnitude weaker than the current Cosmic Microwave Background radiation.
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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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- There’s another time in the Universe, not in the future but in the distant past, when the Universe was also dominated by something other than matter and radiation: during cosmic inflation.
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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.
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- 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 Kelvin, which is 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.
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- 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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- No matter how clearly in your mind you’re capable of envisioning an empty Universe with nothing in it, that picture simply does not conform to reality.
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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.
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- The Universe has never been completely empty, and so long as dark energy doesn’t decay entirely away, it never will be. Wrap your mind round that.
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- March 20, 2021 UNIVERSE - why is it expanding? 3099
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--------------------- --- Sunday, March 21, 2021 ---------------------------
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