- 3420 - PHYSICS - erases the Universe? Of all the questions humanity has ever pondered, perhaps the most profound is, “where did all of this come from?” The idea that we could find the answers by examining the Universe itself was foreign until recently, when scientific measurements began to solve the puzzles that had stymied philosophers, theologians, and thinkers alike.
------------------- 3420 - PHYSICS - erases the Universe?
- The 20th century brought us General Relativity, quantum physics, and the Big Bang, all accompanied by spectacular observational and experimental successes. These frameworks enabled us to make theoretical predictions that we then went out and tested, and they passed with flying colors while the alternatives fell away.
-
- But the “Big Bang” left some unexplained problems that required us to go farther. When we did, we found an uncomfortable conclusion that we’re still reckoning with today: any information about the beginning of the Universe is no longer contained within our observable cosmos.
-
- Looking back to greater distances means looking back in time. The stars and galaxies we see today didn't always exist, and the farther back we go, the closer to more unknowns. In the 1920s our conception of the Universe changed forever as two sets of observations came together in perfect harmony.
-
- Scientist Vesto Slipher had begun to measure spectral lines, emission and absorption features, of a variety of stars and nebulae. Because atoms are the same everywhere in the Universe, the electrons within them make the same transitions: they have the same absorption and emission spectra.
-
- But a few of these nebulae, the spirals and ellipticals in particular, had extremely large redshifts that corresponded to high recession speeds, faster than anything else in our galaxy.
-
- Starting in 1923, Edwin Hubble and Milton Humason began measuring individual stars in these nebulae, determining the distances to them. They were far beyond our own Milky Way, millions of light-years away in most instances.
-
- When you combined the distance and redshift measurements together, it all pointed to one inescapable conclusion that was also theoretically supported by Einstein’s General theory of Relativity, the Universe was expanding. The farther away a galaxy is, the faster it appears to recede from us.
-
- If the Universe is expanding today, that means that all of the following must be true:
-
------------------------ The Universe is getting less dense, as the fixed amount of matter in it occupies larger and larger volumes.
-
------------------------ The Universe is cooling, as the light within it gets stretched to longer wavelengths.
-
------------------------ Galaxies that aren’t gravitationally bound together are getting farther apart over time.
-
- Those are some remarkable and mind-bending facts, as they enable us to extrapolate what’s going to happen to the Universe as time marches inexorably forwards. But the same laws of physics that tell us what’s going to happen in the future can also tell us what happened in the past, and the Universe itself is no exception.
-
------------------------ If the Universe is expanding, cooling, and getting less dense today, that means it was smaller, hotter, and denser in the distant past.
-
- Radiation, matter, and dark energy/inflation energy densities change with expansion. Matter ,both normal and dark, and radiation become less dense as the Universe expands
The big idea of the Big Bang was to extrapolate this back as far as possible: to ever hotter, denser, and more uniform states as we go earlier and earlier.
-
------------------------ More distant galaxies should be smaller, more numerous, lower in mass, and richer in hot, blue stars than their modern-day counterparts,
-
------------------------ There should be fewer and fewer heavy elements as we look backwards in time,
-
------------------------ There should come a time when the Universe was too hot to form neutral atoms and a leftover bath of now-cold radiation that exists from that time,
-
----------------------- There should even come a time where atomic nuclei were blasted apart by the ultra-energetic radiation leaving a relic mix of hydrogen and helium isotopes.
-
- All four of these predictions have been observationally confirmed, with that leftover bath of radiation, originally known as the “primeval fireball” and now called the cosmic microwave background discovered in the mid-1960s often referred to as the smoking gun of the Big Bang.
-
- You might think that this means that we can extrapolate the Big Bang all the way back, arbitrarily far into the past, until all the matter and energy in the Universe is concentrated into a single point.
-
- The Universe would reach infinitely high temperatures and densities, creating a physical condition known as a “singularity“: where the laws of physics as we know them give predictions that no longer make sense and cannot be valid anymore.
-
- The Universe began with a Big Bang some finite time ago, corresponding to the birth of space and time, and that everything we’ve ever observed has been a product of that aftermath. For the first time, we had a scientific answer that truly indicated not only that the Universe had a beginning, but when that beginning occurred. In the words of Georges Lemaitre, the first person to put together the physics of the expanding Universe, it was “a day without yesterday.”
-
- Only, there were a number of unresolved puzzles that the Big Bang posed, but presented no answers for:
-
--------------------- Why did regions that were causally disconnected, had no time to exchange information, even at the speed of light, have the same temperatures as one another?
-
--------------------- Why were the initial expansion rate of the Universe which works to expand things and the total amount of energy in the Universe which gravitates and fights the expansion perfectly balanced early on to more than 50 decimal places?
-
--------------------- Why, if we reached these ultra-high temperatures and densities early on, are there no leftover relic remnants from those times in our Universe today?
-
- Throughout the 1970s, the top physicists and astrophysicists in the world worried about these problems, theorizing about possible answers to these puzzles. Then, in late 1979, a young theorist named Alan Guth had a spectacular realization that changed history.
-
- The new theory was known as “cosmic inflation“, and postulated that perhaps the idea of the Big Bang was only a good extrapolation back to a certain point in time, where it was preceded by this inflationary state. Instead of reaching arbitrary high temperatures, densities, and energies, inflation states that:
-
----------------------------- The Universe was no longer filled with matter and radiation,
but instead possessed a large amount of energy intrinsic to the fabric of space itself,
which caused the Universe to expand exponentially where the expansion rate doesn’t change over time,
-
---------------------------- Which drives the Universe to a flat, empty, uniform state,
until inflation ends. When it ends, the energy that was inherent to space itself, the energy that’s the same everywhere, except for the quantum fluctuations imprinted atop it, gets converted into matter and energy, resulting in a hot Big Bang.
-
- Theoretically, this was a brilliant leap, because it offered a plausible physical explanation for the observed properties the Big Bang alone could not account for. Causally disconnected regions have the same temperature because they all arose from the same inflationary “patch” of space.
-
- The expansion rate and the energy density were perfectly balanced because inflation gave that same expansion rate and energy density to the Universe prior to the Big Bang. And there were no left over, high-energy remnants because the Universe only reached a finite temperature after inflation ended.
-
- Inflation also made a series of novel predictions that differed from that of the non-inflationary Big Bang, meaning we could go out and test this idea. As of 2020, we’ve collected data that puts four of those predictions to the test:
-
------------------------- The Universe should have a maximum, non-infinite upper limit to the temperatures reached during the hot Big Bang.
-
------------------------ Inflation should possess quantum fluctuations that become density imperfections in the Universe that are 100% adiabatic with constant entropy.
-
----------------------- Some fluctuations should be on super-horizon scales, fluctuations on scales larger than light could have traveled since the hot Big Bang. Those fluctuations should be almost, but not perfectly, scale-invariant, with slightly greater magnitudes on large scales than small ones.
-
- With data from satellites like COBE, WMAP, and Planck, we’ve tested all four, and only inflation and not the non-inflationary hot Big Bang yields predictions that are in line with what we’ve observed.
-
- This means that the Big Bang wasn’t the very beginning of everything; it was only the beginning of the Universe as we’re familiar with it. Prior to the hot Big Bang, there was a state known as cosmic inflation, that eventually ended and gave rise to the hot Big Bang, and we can observe the imprints of cosmic inflation on the Universe today.
-
- Only for the last tiny, minuscule fraction of a second of inflation. Only for the final 10^-33 seconds of it can we observe the imprints that inflation left on our Universe.
-
- Inflation is like pressing the cosmic “reset” button. Whatever existed prior to the inflationary state gets expanded away so rapidly and thoroughly that all we’re left with is empty, uniform space with the quantum fluctuations that inflation creates superimposed atop it.
-
- When inflation ends, only a tiny volume of that space, somewhere between the size of a soccer ball and a city block. will become our observable Universe. Everything else, including any of the information that would enable us to reconstruct what happened earlier in our Universe’s past, now lies forever beyond our reach.
-
- It’s one of the most remarkable achievements of science of all that we can go back billions of years in time and understand when and how our Universe came to be this way.
-
- But like many adventures, revealing those answers has only raised more questions. The puzzles that have arisen this time, however, may truly never be solved. If that information is no longer present in our Universe, it will take a revolution to solve the greatest puzzle of all, where did all this come from? God only knows.
-
- For the first 7.8 billion years, the Universe unfolded exactly as scientists would have expected in the aftermath of the Big Bang. The Universe started off expanding at a tremendously rapid rate, while the gravitational influence of all the matter and energy worked to slow that expansion down.
-
- The expanding Universe was a race between these two contenders: the initial expansion, which drives the material in the Universe apart, and gravitation, which works to pull everything back together.
-
- But, about 6 billion years ago, the unexpected occurred. The initial expansion didn’t win; gravitation didn’t win; nor did the two add up to some perfectly balanced tie. Instead, an extra effects began to show up, as though some new phenomenon was causing the expansion rate to speed up once again.
- This phenomenon, known today as “dark energy“, was first uncovered back in the 1990s, and the evidence for it has grown to reach overwhelming proportions today. It leads to an unsettling, empty, lonely fate for our Universe.
-
- When we look back at a distant object in the Universe, we’re not seeing it exactly as it is today. We’re not even seeing it exactly as it was when the light was emitted from it, either. Instead, what we actually observe is a combination of two effects:
-
----------------------------- The light emitted from the source, minus whatever light was absorbed between the source and our eyes,
-
---------------------------- How that light is shifted by all the sources of motion, mass, gravitation, and the expanding fabric of the Universe itself, as measured relatively between the source and the observer
-
- That second effect is tremendously informative, because it tells us that if we can understand how mass, gravitation, motion, and emission and absorption take place, we can use all of the leftover information to reconstruct how the Universe has expanded over its history.
-
- By measuring sources at different distances from us with different light-travel times to our eyes we can learn how the Universe has expanded over its history.
-
- This is where that big surprise of dark energy came from, over the most recent 6 billion years, we’ve seen the Universe expand at a different rate than what the known forms of matter and radiation, even including dark matter, would indicate.
-
- It means that either:
-
--------------------------- There’s an extra energy component to our Universe responsible for this, what we call dark energy,
-
--------------------------- The Universe obeys a different law of gravity than General Relativity on large scales and/or at late times, which only becomes apparent after the Universe has aged, expanded, and diluted past a certain critical point.
-
- Either way what we see occurring is the same. On small scales, gravitation can win many individual battles throughout the Universe, creating star clusters, individual galaxies, galaxy groups, and even large galaxy clusters, some of which merge together over time.
-
- On larger scales, however, gravity always loses out. The strength pf gravity falls offas the square of the distance of separation. This is a new force, a new source of energy, a new field, or a new understanding of gravity, that determines the fate of the Universe on the greatest cosmic scales.
-
- Whatever was gravitationally bound by the time the Universe reached 7.8 billion years of age will remain bound for all of cosmic time. But whatever wasn’t yet bound together will never get there; these unbound structures will all expand away from one another, never to meet again.
-
- Because the Universe has dark energy in it, we know that every galaxy within our Local Group, including the Milky Way, Andromeda, the Triangulum Galaxy, both Magellanic Clouds, and perhaps 60 other dwarf galaxies is bound to us.
-
- Given that we can see for 46 billion light-years in all directions, that means that already, only 6 billion years into the era of dark energy dominance, 94% of the presently observable Universe is already permanently unreachable.
-
- Or, at least, it’s unreachable if the following two things are true:
-
------------------------- We’re limited, in how fast we can travel through space, by the speed of light and the laws of Einstein’s relativity.
-
------------------------ That dark energy is consistent with behaving as a cosmological constant, as a form of constant energy inherent to the fabric of space itself.
-
- But either one of those assumptions can be wrong, and there are many different scenarios that can keep the rest of the Universe from speeding away until it’s forever beyond our reach.
-
- If we simply stayed put in our own Milky Way and waited for long enough, the night skies beyond our own Local Group ,or whatever remains of it after all the galaxies have merged together, would be completely empty, with only the fading light from long-gone galaxies to keep us company.
-
- Here are the three most interesting ways we could possibly circumvent dark energy and visit the distant Universe for ourselves:
-
--------------------- 1.) Dark energy evolves over time. The best data we have, from the cosmic microwave background and the large-scale clustering of galaxies, points to dark energy being completely constant over time.
-
- But that isn’t necessarily the case, as many different “variable field” scenarios can lead to dark energy changing strength or even sign over time. If dark energy either becomes weaker or becomes negative, rather than positive, the expansion will slow down and possibly even reverse, making these galaxies reachable again.
-
- Measuring the necessary galaxies to test for this is also one of the major science goals of the “Nancy Roman Telescope“, which is slated to construct and launch as its next astrophysics flagship mission after James Webb.
-
- Right now, our best observations show that dark energy is consistent with a cosmological constant, but with an uncertainty of about 12% on that figure. Roman will give us a measure of dark energy that’s about 10 times more sensitive than our present data, teaching us if dark energy is different than our simple expectations by as little as 1%.
-
---------------------- 2.) Bending or folding space enables us to take a cosmic short-cut. Sick of being limited by the speed of light in your attempts to journey through the Universe? The idea of Star Trek’s “Warp Drive” might still be science fiction, but there’s a real-life scientific possibility of making it a reality: the Alcubierre drive.
-
- In Einstein’s General Relativity, it’s possible to fold, bend, or otherwise distort the fabric of space, enabling a fantastic possibility: compressing the space in front of you at the expense of expanding the space behind you.
-
- If we could make this a reality, we could theoretically compress the space ahead of us, travel through it at a slower-than-light speed, and then arrive at a destination appearing to have traveled faster than light could!
-
- The only downside is to make this theoretical possibility a reality, we’d need some form of negative energy or negative mass to exist. There’s an experiment happening at CERN right now to measure whether antimatter falls down or up in a gravitational field; if it falls up, the Alcubierre Drive might become a reality!
-
------------------------- 3.) Dark energy is bound to inevitably decay. Perhaps dark energy only appears to have a constant energy density for the time being, and that given enough time, it will decay in some fashion.
-
- While much has been made of vacuum decay, or the possibility that an immediate transition will knock the energy inherent to space down to a lower value, destroying the Universe as we know it instantly, there are other forms of decay that are gradual and non-lethal, such as a conversion of energy from one form into another.
-
- It’s possible that this could simply result in the creation of a low density of particles: somewhere around one proton per cubic meter of space, at the cost of virtually eliminating dark energy.
-
- If this occurred, the expansion rate would change dramatically, as the Universe would immediately begin slowing down again. All of the distant galaxies, even the ones that appear unreachable today, would suddenly be within the reach of a relativistic spaceship. At close to the speed of light, we could potentially travel anywhere in the known Universe.
-
- Perhaps our measurements are biased and have led us to an incorrect conclusion, but that would require an enormous number of independent lines of evidence all being biased in the same way.
-
- Perhaps we’ve got the laws of gravity wrong; perhaps we live in a very special and unusual region of the Universe that’s causing us to wrongly conclude that dark energy exists; perhaps there’s a novel force or interaction that exists that we simply haven’t properly identified.
-
- In science, however, we base our conclusions on the full suite of data and evidence we have at our disposal, keeping in mind that they may change over time as we gain new and better information.
-
- The expansion rate is changing over time in a way that requires dark energy as the dominant component in our Universe, and dark energy is consistent with it being a cosmological constant, its energy density doesn’t appear to change with time.
-
- Unless dark energy reveals itself as something different or we find a short-cut through space, the majority of the observable Universe is forever beyond our reach already. You will have to wait to learn what happens next.
-
January 19, 2022 PHYSICS - erases the Universe? 3420
----------------------------------------------------------------------------------------
----- Comments appreciated and Pass it on to whomever is interested. ---
--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email feedback, corrections, request for copies or Index of all reviews
--- to: ------ jamesdetrick@comcast.net ------ “Jim Detrick” -----------
--------------------- --- Thursday, January 20, 2022 ---------------------------
No comments:
Post a Comment