Chapter 1: What is the crisis gripping cosmology?
There's a crisis in the cosmos, and the fate of the entire universe depends on it. We've known that we live in an expanding universe since 1929, when Edwin Hubble first proved galaxies are moving farther and farther apart as time goes on. This expansion has come to underpin our entire theory of cosmic evolution, from the Big Bang to the formation of galaxies.
The key factor is the rate of the expansion of the universe, a value that has become known as the Hubble constant. We thought this value was pretty set, but now, astronomers aren't so sure. Do we stand on the brink of a cosmic revolution, or could it be that our entire model of the universe is wrong? I'm Alex McColgan, and you're watching Astrum.
Join me as we explore the crisis gripping cosmology, revealing how Hubble's greatest discovery has become one of today's most troublesome questions.
Chapter 2: How did Edwin Hubble change our understanding of the universe?
For most of human history, the universe was small. Or at least, we thought it was. Some ancient civilizations imagined Earth to be flat and covered by a dome of stars. The sun and moon rose and set, and the constellations turned overhead.
By the second century, astronomers such as Claudius Ptolemy had envisioned a geocentric universe, with Earth fixed at the centre, and the Sun, Moon, planets and stars all revolving around us. This would be the reigning model for more than a thousand years, until the 1500s, when Nicolaus Copernicus proposed a heliocentric model, where our Earth was one of several planets circling the Sun.
A revolutionary idea. Fast forward to the 18th century, and William Herschel mapped some of the stars in our galaxy, the Milky Way, proving that it was a vast disk-shaped system. As antiquated as it may sound now, astronomers debated for some time whether the Milky Way was the entire universe, or if it was in fact one of several island galaxies in a much larger cosmos.
That is, until 1923, when Edwin Hubble resolved the question once and for all. Peering through the 100-inch Hooker telescope at the Mount Wilson Observatory in California, the world's largest telescope at the time, Hubble was able to resolve individual stars in Andromeda, and within them found his first Cepheid variable.
Using Leavitt's law, named after Henrietta Leavitt, who found more than 2,400 variable stars and discovered that they could be used to measure distances across the universe, Hubble determined that Andromeda was some 900,000 light years from Earth, an astonishing distance. Too far away to be part of the Milky Way, this proved that another galaxy with its own stars existed outside of our own.
Overnight, Hubble widened our view of the universe immeasurably. But he didn't stop there. He would go on to use Levitt's law to identify 23 other galaxies and measure their distances, some as far away as 20 million light years from our planet. Ultimately, Hubble concluded that millions of other galaxies must exist outside of our own, forever changing astronomy.
But Hubble's next revolutionary discovery
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Chapter 3: What is the significance of the Hubble constant?
also needed the work of Vesto Slipher, an astronomer born on a farm in Indiana in 1875. He joined the Lowell Observatory in Flagstaff, Arizona in 1901, and between 1912 and 1925, he made systematic observations of more than 40 spiral galaxies.
At the time, it was known that the light observed from these distant galaxies could be split into two spectral components, and depending on what elements were present in the light source, different patterns would appear. If the object being observed was moving away, then those same patterns would be present, only they would be shifted towards the red end of the spectrum, what we call redshift.
Through years of observation, Slipher found that nearly all galaxies appeared to be moving away from us, but at the time, he didn't have a way to measure the distances to these faraway bodies, let alone their velocity. In 1927, Belgian scientist Georges Lemaitre discovered
proposed a theory that the universe was the same in all directions, and that if Einstein's theory of relativity was right, it must also be expanding. But he had no evidence to support this theory, so it was ignored for the most part. That is, until we bring Hubble back into the picture.
Like most scientists at the time, Hubble was unaware of Lemaitre's theory, or of another similar one proposed a few years earlier by Soviet scientist Alexander Friedman. Instead, from the observatory on Mount Wilson, Hubble made the same observation as Leifer, that galaxies seemed to be moving away from us. But he noticed something that nobody else had spotted before.
The farther away a galaxy was, the more redshift it appeared to have, meaning the faster it was racing away from us, and this was happening in every direction. More observations revealed that almost all galaxies appeared to be moving away from each other, and that the redshift of a galaxy was directly proportional to the distance of the galaxy from Earth. This was a major breakthrough.
It meant that the universe must be expanding. Hubble announced his finding in 1929, and following its publication, it became supporting evidence for Lemaitre's expanding universe theory, which would become known as the Big Bang.
Hubble's initial measurement of the rate of the expansion of the universe, which would become known as the Hubble constant, was roughly 160 km per second per million light years. That's about 500 km per second per megaparsec. However, that number was not quite accurate, and even Hubble himself worked to refine it over his career.
Since then, the Hubble constant has become a fundamental value in cosmology. We've used it to establish the age and the size of the universe, and those numbers are important for many, if not all other cosmological calculations. To narrow in on the most precise value for the Hubble constant, astronomers have used two primary methods. But something about this value isn't adding up.
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Chapter 4: What methods are used to measure the Hubble constant?
It may one day turn out that singularities do not exist in the hearts of black holes at all, but this is the extent of our knowledge so far. Well, whatever it is that lies inside a black hole, it powers our faster than light engine, because like most objects in the universe, it spins. And oh does it spin.
As we travel out from the centre of the black hole, we pass through the event horizon with little fanfare. The event horizon actually cannot be detected locally, although a person outside the black hole might watch you slow down to a complete stop as you travel through it. From your perspective, it actually might seem like time is flowing normally.
Normally that is, until the universe outside the black hole runs its course in an instant, because time outside the black hole is travelling so fast compared to you.
Chapter 5: Why are astronomers confused about the Hubble constant?
This is the essence of relativity, and we talk about it in another of my videos, which you can look at here. In fact, the only evidence you might have that you've passed the event horizon at all is because of something that exists just outside it, the photon sphere.
In a zone just outside the event horizon, there exists a point in space where if a photon enters it at just the right angle, it will enter a perfect orbit around the black hole in much the same way the Moon perfectly orbits the Earth.
This infinitesimally thin zone is known as the photon sphere, and given the number of photons that have flown past black holes in all the millions of years they have existed, it is probably filled with photons. It is quite possible that you would be instantly fried as you pass through this point. However, it is just outside here that we find the zone that interests us, the ergosphere.
This is the zone around a black hole where we can most easily detect its spin, and this is because, in this zone, it is impossible for us not to move. You see, mass affects space. We see this in the curving effect of gravity on the travel of objects through that region of space. However, it might be more accurate to say that mass drags on the space around it.
As it moves through space, it brings a little bit of that space along with it for the ride, and when an object as massive as a black hole spins, there is an effect known as frame dragging. To put it simply, reality around the black hole begins to spin in a whirlpool that cannot be fought against.
Much like a real whirlpool, anything caught within the ergosphere is spun around the black hole, because the frame of reference it sits in is being pulled. Sort of like how a person moves because they are standing on a moving walkway.
The greater the spin of the massive object, the faster this happens, and in the ergosphere this can occur at a speed so fast that by the event horizon space is moving faster than the speed of light. you would need to travel faster than the speed of light in the opposite direction just to stay at a relative standstill from the point of view of the outside observer, which of course you cannot do.
But isn't this against the laws of physics? Doesn't Einstein say that nothing can travel faster than the speed of light? The answer to that is yes, but black holes have found an interesting loophole. You see, this rule only applies locally. Right where you are, in your frame of reference, nothing can go faster than the speed of light.
But thanks to relativity, it is possible for frames of reference to move away from each other so fast that objects in them appear to be breaking this light barrier from your point of view. But if you moved next to them and entered their frame of reference, they would seem to slow down and would start obeying the laws of physics again.
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Chapter 6: How might dark energy influence the expansion of the universe?
Let's fill them in now. Of course, if you detect particles using the same detector on both particles, you'll get a contrasting result because they're entangled. But we're not interested in these results. Classical physics and quantum physics both agree on this, so let's remove them.
What are the odds that two different detectors for particle A and B will see the same result, and what are the odds they'll differ? Remember, quantum physics expected it to be 50-50. Particles are making up their values on the spot, and so it's perfectly random which they'll choose, as they aren't confined by the opposites rule here.
But in this table, classical physics says that contrasting results only happen a third of the time. The other times, they're either both up or both down. If we do this many times, assigning different directions each time and ignore exceptions, for instance where the spins of the particles are all up up up or down down down, once you crunch the numbers,
The important thing to take from all of this is that according to the maths, classical physics predicts a matching outcome 55% of the time, while quantum physics continues to simply predict 50%. Pretty table, be damned. This percentage difference was the key. By quantumly entangling particles, and running this test over and over again, you could now see which percentage was correct.
And it turned out the winner was quantum physics. Particles were just apparently making up their spin results on the spot. Which is spooky. Because not only does that call into question our perceptions of reality itself, but that also means that the moment one particle decided on its spin result, its quantum entangled partner instantly knew that that decision had happened.
You could test both particles at once. no matter the distance, and this same result would come back. Somehow, information had travelled from the one particle to the other in no time at all, far faster than light itself. So already, something strange was going on here. This result disproved Einstein's predictions, and showed that some information does seem to go faster than light.
we can take this one step further and have information going back in time. There is another experiment known as the delayed choice test. Its primary purpose was to explore the fundamental nature of light, whether it was a wave or a particle, and to figure out when it decided to be one or the other. Experiments like the double slit experiment had done this in the past to mixed results,
Sometimes light behaved in a wave-like manner, creating interference patterns on detectors that could only happen if a wave was interfering with itself. But sometimes it behaved like a particle, hitting only a single point on a detector. But most baffling of all, it seemed to change which it behaved like depending on whether you were observing its path through space or not.
If it could go through multiple paths, and no one was watching to see which it did go through, light simply went through both, like a wave. But observed, it went through just the one, like a particle. This result was baffling enough, and deserves a video on its own, but in 2006 a number of scientists took it one step further by asking an interesting question.
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Chapter 7: What is the Hubble Tension and why does it matter?
When the second beam splitter is present, the light produces an interference pattern, indicating that the single photon went down both paths, ultimately bumping into itself before moving on to both detectors. This seems like strong evidence that light is a wave – it certainly behaves like one here – But what happens if you remove the second beam splitter?
Suddenly, you know which path the light travelled down. If light arrives at the top detector, it must have arrived from path 1. If it arrives at the side detector, it must have come along path 2. And something about this knowledge spooks the light. It stops going down both paths, and suddenly, each photon only arrives at one detector. Here's the question.
What happens if you insert the beam splitter after the photon has already started down either one or both routes? This is why the test is called delayed choice. If you delay choosing how exactly you intend to detect the photon, whether by knowing which part it came down, or making that ambiguous to you, what happens to the light? What happens is a very strange thing.
When this experiment was performed, it was done multiple times, with the beam splitter randomly being inserted or not, but always being inserted after the photon had entered one or both paths. And yet, the results came back unequivocal. If the beam splitter was present, the photon suddenly and seemingly retroactively stopped picking a path.
If the beam splitter was removed, the photon seemingly knew it would later be detected and picked a specific path to accommodate. Somehow, the beam splitter being added or removed in the future changed what the photon did in the past. So, what is happening here? Is it really true that particles somehow saw the future? Did the experiment cause information to be sent back into the past?
Or is there some other principle at play here that explains this whole thing, that accounts for the instant transmission of information between quantum particles, and allows it to be perfectly rational that light could travel down one path or both at the same time? Personally, I'm inclined to think that this is more likely.
We clearly don't understand what is happening here, but it must be admitted, if we don't understand what is happening, there's nothing to say that causality isn't being ignored. In some way, maybe on the quantum level, time really is more fluid than it is up here in the larger universe. Maybe space and time simply do not apply down there.
And maybe one day, someone will be able to come up with a theory that allows all these strange phenomena to finally make some sense. Until then, we'll just have to keep asking the same question. Can information travel backwards in time? Until then, we'll just have to all agree on one thing. Quantum physics is strange. A massive thank you to our Astronauts on Patreon.
This video had no sponsors, but it was still made possible thanks to the hundreds of members we have there. Link is in the description to join our growing community. Patreon is where Astrum truly takes shape. A place for people who love space, who want to see these videos keep improving and reaching more curious minds. Every new member keeps the channel focused on what really matters.
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