So much about our "known" universe remains a mystery; such things as black holes, dark matter, and dark energy seem to operate in ways that astronomers are only beginning to understand-- and much of that understanding is theoretical, nebulous, based in little more than speculation. The aim of this blog post is to shed some light in places where light literally cannot reach, and where the light of clearer understanding has not yet shone. I will begin by discussing the natures of black holes and their various bizarre properties, before delving into the topics of dark matter and energy and the ways in which they interact-- or don't-- with normal matter and energy. Now, I invite you all to come with me on a journey into the most warped and distant corners of space-time, where we will begin to unravel the tangled threads that make up the mystery that is our universe.
Black Holes
Basic Properties of Black Holes
A black hole, though its name may imply otherwise, is not truly black; nor is it a hole in the true sense of the term. A black hole is a region of space around which the pull of gravity is so strong that not even light can escape it. The very "edge" of a black hole, or the event horizon or Schwarzschild radius, has an escape velocity equal to the speed of light; any matter that crosses this radius collapses into a single point known as the singularity. The space surrounding the black hole is quite literally bent, as is starlight; the mass of the black hole acts like a lens, warping the straight strands of the fabric of space-time into what could almost be compared to wells. These "potential wells" make it more difficult for light to climb out (assuming it does not cross the event horizon), leading to its redshifting away.
- A minor note: while black holes may be considered black because light cannot escape them and radiation is not given off from them, black holes do, in fact, "leak" a form of radiation known as Hawking radiation, which will be discussed in greater depth later on. For more information about Hawking radiation, check out Chris Shaffrey's post here: http://as151.blogspot.com/2016/11/hawking-radiation-and-evaporation-of.html
Time also passes more slowly near the event horizon of a black hole, and tidal forces exert immense pressure there. For a more detailed look at the properties of black holes, check out Kylie's post here: http://as151.blogspot.com/2016/11/black-holes-daunting-and-dramatic-but.html
White Holes and Wormholes
White holes are a purely theoretical concept; they are, in essence, the "anti-black-hole." The idea is that if a black hole is something whose gravity is strong enough to pull all matter into it and collapse it into a single point, then a white hole is something identical to a black hole, but without mass. All matter that comes near its event horizon is expelled out into space; nothing can enter. More information can be found here: http://www.universetoday.com/122715/what-are-white-holes/
Although white holes should not, according to the laws of physics, be able to actually exist (they would collapse as soon as even the smallest speck of matter enters the region of the event horizon), some theoreticians believe wholly in their existence; that existence, they claim, is predicted by the solution to the Schwarzschild equation. If you recall, the Schwarzschild radius is the point around a black hole at which the escape velocity is equal to the speed of light. This radius becomes the term r^2 in the Schwarzschild solution, which is the solution to the set of the Einstein field question.
The fact that the solution allows for negative as well as positive square root has led some astronomers to believe that the Schwarzschild model of the universe was meant to account for "a black hole [the positive root], a white hole [the negative root], and two Universes connected at their event horizons by a wormhole" (http://casa.colorado.edu/~ajsh/schww.html). The wormhole, of course, is a sort of bridge (known as an Einstein-Rosen bridge) between two (or more) different universes of space-- and time. Theoretically, if the vacuum-like black hole is the "entrance" to the wormhole, and the fountain-like white hole is the "exit," then the wormhole connecting the two will allow for high-velocity travel between these entrance and exits points. For more detailed information, check out these guys at www.physicsoftheuniverse.com: http://www.physicsoftheuniverse.com/topics_blackholes_wormholes.html
Quantum Entanglement and the Firewall Paradox
So what, you may be wondering, does the wormhole theory have to do with anything? Well, I may have neglected to mention that it was Einstein who came up with that theory; he and Nathan Rosen proposed that idea in 1935, along with the more elusive theory of ER = EPR (P for the extra collaborator Boris Podolsky). ER = EPR is a theory describing "spooky actions at a distance," or the interactions between quantum particles. Though these ideas were thought to be unconnected for about 80 years, that view is slowly beginning to change-- all because of those spooky little quantum particles, sewing the fabric of space-time together.
K.C. Cole, contributor to Quanta magazine, is likely better able to explain quantum entanglement than I am, so for the sake of your curiosity I'll include the link to her article about entanglement at the end of this section. Essentially, though, entanglement occurs when two quantum particles become tied to,each other; what happens to one happens to the other, no matter how far apart the particles are. One one kind of entanglement, however, can occur at a time, which is where the firewall comes in.
News flash: information that falls into a black hole isn't lost. Instead, it is trapped at the event horizon, which encoded all of the information going into the black hole and scrambling it, making it irretrievable in its previous state. The problems really start to occur when, in order to contrive a smooth and continuous event horizon surface for the black hole, particles falling in are entangled with those that escape. In the meantime, those that escape are also entangled with ancient particles that left the hole years ago and scattered among the Hawking radiation. Since only one state of entanglement can occur at a time, one of these links has to go, and it is the first one that gives. The broken nature of space-time inside the black hole, however, implies the existence of an immense buildup of energy: the firewall. Yeah, that doesn't sound practical to me, either.
That paradox, however, is where ER = EPR (EPR for short) can save the day. EPR entanglement is an entirely new sort of entanglement that links parts of the fabric of space-time together-- more specifically, the parts between the event horizon of a black hole and the surrounding space-- with wormholes. Those particles inside and out of the horizon, entangled as they would be, would be one and the same, and the energetic disturbance in the black hole would not have to exist. This theory wraps everything up rather nicely. Sad, that it's but a theory still.
Dark Matter and Dark Energy
What is Dark Matter?
Dark matter comprises about 27% of the universe, and though it's less of a mystery than dark energy is, much about it still remains unknown. It gives off no radiation, it is not made up of dark clouds of baryonic (normal) particles, and it does not interact with ordinary matter. In fact, dark matter only interacts with gravity. The leading theory is that it is comprised of some massive exotic particle such as a WIMP (weakly interacting massive particle), but halos of dark matter actually surround galaxies, their radii extending out hundreds of thousands of light-years. Because dark matter is so massive, it cannot cool itself down enough to collapse in on itself because the vacuum of space prevents cooling by conduction and convection, and the non-radiative dark matter cannot radiate away photon energy. In fact, it heats up so much trying to that the thermal pressure repels the force of gravity, and therefore the dark matter remains extended so far outward. Dark matter can also, according to new research, form galaxies of its own; read more here: https://www.quantamagazine.org/20160927-ultra-diffuse-galaxy-dragonfly-44/
Dark Matter and Black Holes
Dark matter may be formed from WIMPs, but another theory floating about is that it is actually residue from the earliest, or "primordial" black holes. Cosmic infrared background images show a curious patchiness in the radiation glows that is matched by periodic irregularities in X-ray radiation background findings-- irregularities that could only be produced in both wavelengths by black holes. Further studies have actually been able to detect primordial black holes, which formed in areas where the sounds waves of the Big Bang were densest, via gravitational waves produced when these black holes merged shortly after the Big Bang.
Now, what does all this have to do with dark matter? Another interesting observation made by the teams that worked with these primordial black holes was that the cosmic infrared background and X-ray background were enormous-- much larger than they should have been from mere star radiation or gas falling into the black holes. It's known that dark matter clumped into early halos, and galactic halos eventually collapsed and formed within those, but if dark matter was made of black holes, many more halos would have been created-- enough to potentially explain the unexpectedly large size of the radiation backgrounds.
Bear in mind, of course, that the primordial black hole theory is merely that at the moment-- a theory, that has not been proven. For more information, check this out: http://www.space.com/33122-dark-matter-black-hole-connection.html
What is Dark Energy?
Even less is known about dark energy than about dark matter. We know that it comprises about 68% of the universe, and that's about it...or is it? That 68% was calculated based on the expansion rate of the universe. That rate has been increasing since the time of the Big Bang, but it really spiked about 7.5 billion years age. Something was pushing galaxies apart more and more quickly, something that literally increased the amount of space between them. That "something" was dark energy.
Currently there are four major theories attempting to explain precisely what dark energy is. More detailed explanations can be found here-- https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy-- but in brief, they involve: dark energy being a property of space, the quantum theory of matter, quintessence, and Einstein's theory of gravity being incorrect.
Property of space
Einstein was the one to suggest that empty space was not, in fact, empty, but rather had its own properties. One of his theories suggested that there was not a finite amount of space, but rather, that more space could come into existence. Another theory-- his gravitational theory involving a cosmological constant-- suggested that space had its own energy. The two properties were intrinsically linked; as more space came into existence, it would gain more energy, causing the universe to expand more quickly.
The Quantum Theory of Matter
The quantum theory suggests that "empty" space is actually filled with particles called virtual particles, or "temporary" particles, that constantly form and disappear. The issue with this theory is that the number derived from trying to calculate how much energy this continuous reformation would give space was far too large to be feasible.
Quintessence
Astronomers and physicists seem to love paradoxes and theories and, above all, new, weird, and unexplained stuff. Quintessence is just one of many things comprising this stuff. The idea is that some new, "dynamical" energy field or fluid, called quintessence, fills all of space and behaves in the opposite way that normal matter and energy do regarding its effect on expansion. However, if it exists, no one knows what it is, or why it exists. What is known is that because it's described as a dynamic force, it must change with time, completely contradicting Einstein's idea of dark energy being a cosmological "constant."
Einstein Was Wrong???
If Einstein's theory of gravity was incorrect, our understanding of both universal expansion and the behavior of normal galactic matter would be turned completely upside-down. Observing the way galaxies formed after abandoning previous ideas might produce a solution to the dark energy mystery...if only there was a new theory actually in place. Thus far, no theory put forth has been able to explain everything astronomers have observed.
Space is vast, and expanding at a rapidly increasing rate. Just as space expands, so too does our knowledge of it. At the same time, as we discover more and more about our universe, we seem to be uncovering more mysteries that challenge the very foundations of science as we understand it. There's no telling how much more research will have to be done to gain even a bit of insight into the strange workings of such things as black holes, dark matter, and dark energy, but in any case, the field is positively abuzz trying to carry this research out. I hope you've enjoyed your flash tour of the universe's little-knowns. Perhaps I've even inspired you to try and solve those mysteries. Certainly I've inspired myself!
No comments:
Post a Comment