This goes out to my boi TexasDeac10 who asked for this the other day.
Most of our Universe is missing. Yes, you read that correctly. Ordinary matter only makes up around 4.6% of the observable universe. The other 95.4% is made up of several different things. Keep in mind, these names are placeholders. We don’t know what to call it and since we can’t see it, the moniker “dark” is often used.
Normal Matter
This is all of the things that you can see. Some examples: Felix the dog, Tim Duncan, the Joel, the Earth, the Sun, Betelguese, and Alpha Centauri to name a few. If it emits light, it is normal matter. It is mind boggling to me that the universe is only made up of 4.6% of this stuff. More than 95.4% of the universe is made up of something that has never been detected in a laboratory!
Dark Matter
As I mentioned earlier, in order for us to be able to detect something in space, it must emit light. There could be a “dark” object residing out next to Mars and we would not be able to see it. We would only be able to infer its presence from its gravitational effects on Mars and other surrounding planets or by taking a ship out there and visiting it. These same gravitational effects are what tipped Fritz Zwicky (transformers anyone?) off in the 30s to something strange about the make-up of galaxies he observed. He used observations of a galaxy cluster, determined the average mass of the galaxies within, and then looked at the actual luminosity compared to the predicted. Wow! It turns out that those galaxies should have been many, many times more luminous. This is the first instance of the idea of dark matter and its effects have since been observed across the universe.
One would expect most of the mass of a galaxy to reside within its central bulge due to gravitational effects of rotation. Actually, it is the exact opposite, in order for galaxies to exist as we see them there must be vast quantities of dark matter existing within the outer reaches of galaxies (the halo.) There are a few other observations that indicate the existence of dark matter, but the most important is mentioned above. We can’t see this stuff, so what is it?
For our discussion, we can divide dark matter into two possible categories. Baryonic dark matter, or matter made up of atoms, resides in objects that do not emit light. They either devolved into a dark object or never gave off electromagnetic radiation at all. The dark object residing next to Mars would be an example of baryonic dark matter. Black holes, neutron stars, white dwarfs, planets and brown dwarfs are all possibilities for baryonic dark matter. These guys are affectionately called MACHOs, short for massive astrophysical compact halo objects. An interesting note, in 2005, astronomers claimed to have discovered a dark galaxy in the Virgo cluster. It has almost 100 times more dark matter than our own Milky Way and appears to contain 1000 times more dark matter than hydrogen, the most common element in the universe.
(we don't actually know what brown dwarfs look like, so this is an artists representation. Its pretty big, though)
Common sense tells us that there can’t be enough baryonic dark matter to account for the enormous discrepancy we see between ordinary matter and dark matter in the universe. Thusly, we need another vehicle to describe the rest of the distribution. What else could be causing the mass discrepancies in galaxies? One of the most common non-baryonic particles is called a neutrino. As you sit here reading this, a million million neutrinos are passing through your body at very close the speed of light. These are by products from the fusion occurring at the sun’s core. Could these make up the other portion of the dark matter distribution in our universe? It is unlikely. These particles move very fast and are easily able to escape a galaxies gravitational pull. And on top of that, they rarely interact with normal matter and are immensely hard to detect. I once heard that it would take several hundred miles of lead to stop a neutrino in its tracks. I’m not 100% sure if this is true or not, but it seems plausible.
If not neutrinos, what else? Could there be some type of particle that exhibits some of the same properties as a neutrino, but is much more massive? If there were such a thing, it would still be extremely difficult to detect, BUT each particle would be much more attractive due to its larger mass. This leads us to another cool acronym. WIMPs, or, weakly interacting massive particles. These guys would be completely invisible in all wavelengths of light and would thus fulfill our dark moniker. Most evidence points to these particles making up the majority of the dark matter in our universe.
The most convincing evidence resides in the mechanics of galaxy formation. As galaxies began to form out of a cloud of dust and gas, they would have combined mostly dark matter with hydrogen and helium. The individual gas particles would have collapsed to form the rotating disk around the core because this normal matter can lose orbital energy. Some of the light we see from these galaxies is due to this orbital energy being converted into radiative energy in the form of photons. WIMPs rarely interact with normal matter and cannot give off photons, so they would have remained in their ordinary orbits far out in the halo of galaxies while the normal matter was partying around the core. This agrees with our observations that most of the mass of a galaxy resides in its halo!
As we speak, there are multiple experiments set up in the deep, abandoned mines that are designed to detect these WIMPs. It is only a matter of time!
There are a few other possible explanations out there such as alternate theories of gravity, but they don’t really hold much weight within the scientific community.
Dark Energy
All these dark things can get confusing. You wouldn’t expect energy to give off any light, or to be visible at all. Why would they call it dark? Well, there really isn’t a reason. The name just stuck, it could have been called deacvision7 energy and it may have stuck as well.
I’ve talked about this in previous posts, but in 1929, Edwin Hubble determined from years of observations, that the universe was expanding. Looking out from Earth, every single galaxy that isn’t gravitational bound to our home cluster (Virgo) is receding away from us at a velocity proportional to its distance from us. This has enormous implications on our understanding of the large scale structure of the universe and its origins.
Why is the universe expanding and why is it doing so at an ever quickening pace? Observations by the hubble space telescope tell us that early on in the life of the universe, galaxies were receding much slower than they are now. Dark energy is our explanation for this expansion. In fact, dark energy accounts for 70% of the total energy density of the universe! This is one of the most puzzling topics in cosmology today and many bright minds are trying to figure it out. Needless to say, there are several different hypotheses about the properties of dark energy.
Could it be a property of space? None other than Albert Einstein discovered that it is possible for more space to pop into existence. Further, his theory of gravity proposed that empty space has its own energy. As this energy is a property of space itself and would not be minimized by expansion as more space pops into existence. Thus, as more and more space was created, more energy would come into existence. Unfortunately, this explanation for dark energy falls flat. Its predictions for the total energy of space itself come out to be 120 orders of magnitude off. Bummer!
One fledgling hypotheses states that dark energy is a type of energy fluid or field that fills all of space, a quintessence. I don’t really know anything about this one, but it doesn’t seem likely. Who knows though?
Could Einstein’s general relativity be wrong? This does not seem likely either, every year we discover more and more observational evidence that general relativity is correct. In fact, on May 27, 2011, NASA’s gravity probe-b verified the geodetic effect, or the amount that space is warped by Earth and the frame dragging effect where space and time are pulled by the rotation of the Earth. These are two effects that were predicted by general relativity. As observations like these continue to be recorded, it becomes less likely that we will toss out relativity.
In the end, we just aren’t sure what dark energy might be. You can rest assured that some very smart people are working on this, though J
Dark Flow
This is my favorite “dark” topic. Galaxies in the Centaurus supercluster are moving away in accordance with Hubble flow, but there are variations in their velocities which indicate that another force is acting on them, causing them to recede even faster. Whatever is attracting these galaxies is called the great attractor and it is gravitationally equivalent to several tens of thousands of Milky Ways. Unlike the standard expansion of the universe, this velocity is constant. The matter in our observable universe cannot produce such a gravitational force.
The scientists who proposed this anomaly believe that the culprit could be something which resides outside of the observable universe. Several ideas have been put forth, but the most intriguing to me is that this dark flow could be the imprint of an alternate universe on our own. Plausible? Maybe. Awesome? Yes!
As always, questions are welcome. I love talking about this stuff.
In a shameless show of self-promotion, I'll pos rep anyone who asks a question.
Most of our Universe is missing. Yes, you read that correctly. Ordinary matter only makes up around 4.6% of the observable universe. The other 95.4% is made up of several different things. Keep in mind, these names are placeholders. We don’t know what to call it and since we can’t see it, the moniker “dark” is often used.
Normal Matter
This is all of the things that you can see. Some examples: Felix the dog, Tim Duncan, the Joel, the Earth, the Sun, Betelguese, and Alpha Centauri to name a few. If it emits light, it is normal matter. It is mind boggling to me that the universe is only made up of 4.6% of this stuff. More than 95.4% of the universe is made up of something that has never been detected in a laboratory!
Dark Matter
As I mentioned earlier, in order for us to be able to detect something in space, it must emit light. There could be a “dark” object residing out next to Mars and we would not be able to see it. We would only be able to infer its presence from its gravitational effects on Mars and other surrounding planets or by taking a ship out there and visiting it. These same gravitational effects are what tipped Fritz Zwicky (transformers anyone?) off in the 30s to something strange about the make-up of galaxies he observed. He used observations of a galaxy cluster, determined the average mass of the galaxies within, and then looked at the actual luminosity compared to the predicted. Wow! It turns out that those galaxies should have been many, many times more luminous. This is the first instance of the idea of dark matter and its effects have since been observed across the universe.
One would expect most of the mass of a galaxy to reside within its central bulge due to gravitational effects of rotation. Actually, it is the exact opposite, in order for galaxies to exist as we see them there must be vast quantities of dark matter existing within the outer reaches of galaxies (the halo.) There are a few other observations that indicate the existence of dark matter, but the most important is mentioned above. We can’t see this stuff, so what is it?
For our discussion, we can divide dark matter into two possible categories. Baryonic dark matter, or matter made up of atoms, resides in objects that do not emit light. They either devolved into a dark object or never gave off electromagnetic radiation at all. The dark object residing next to Mars would be an example of baryonic dark matter. Black holes, neutron stars, white dwarfs, planets and brown dwarfs are all possibilities for baryonic dark matter. These guys are affectionately called MACHOs, short for massive astrophysical compact halo objects. An interesting note, in 2005, astronomers claimed to have discovered a dark galaxy in the Virgo cluster. It has almost 100 times more dark matter than our own Milky Way and appears to contain 1000 times more dark matter than hydrogen, the most common element in the universe.
(we don't actually know what brown dwarfs look like, so this is an artists representation. Its pretty big, though)
Common sense tells us that there can’t be enough baryonic dark matter to account for the enormous discrepancy we see between ordinary matter and dark matter in the universe. Thusly, we need another vehicle to describe the rest of the distribution. What else could be causing the mass discrepancies in galaxies? One of the most common non-baryonic particles is called a neutrino. As you sit here reading this, a million million neutrinos are passing through your body at very close the speed of light. These are by products from the fusion occurring at the sun’s core. Could these make up the other portion of the dark matter distribution in our universe? It is unlikely. These particles move very fast and are easily able to escape a galaxies gravitational pull. And on top of that, they rarely interact with normal matter and are immensely hard to detect. I once heard that it would take several hundred miles of lead to stop a neutrino in its tracks. I’m not 100% sure if this is true or not, but it seems plausible.
If not neutrinos, what else? Could there be some type of particle that exhibits some of the same properties as a neutrino, but is much more massive? If there were such a thing, it would still be extremely difficult to detect, BUT each particle would be much more attractive due to its larger mass. This leads us to another cool acronym. WIMPs, or, weakly interacting massive particles. These guys would be completely invisible in all wavelengths of light and would thus fulfill our dark moniker. Most evidence points to these particles making up the majority of the dark matter in our universe.
The most convincing evidence resides in the mechanics of galaxy formation. As galaxies began to form out of a cloud of dust and gas, they would have combined mostly dark matter with hydrogen and helium. The individual gas particles would have collapsed to form the rotating disk around the core because this normal matter can lose orbital energy. Some of the light we see from these galaxies is due to this orbital energy being converted into radiative energy in the form of photons. WIMPs rarely interact with normal matter and cannot give off photons, so they would have remained in their ordinary orbits far out in the halo of galaxies while the normal matter was partying around the core. This agrees with our observations that most of the mass of a galaxy resides in its halo!
As we speak, there are multiple experiments set up in the deep, abandoned mines that are designed to detect these WIMPs. It is only a matter of time!
There are a few other possible explanations out there such as alternate theories of gravity, but they don’t really hold much weight within the scientific community.
Dark Energy
All these dark things can get confusing. You wouldn’t expect energy to give off any light, or to be visible at all. Why would they call it dark? Well, there really isn’t a reason. The name just stuck, it could have been called deacvision7 energy and it may have stuck as well.
I’ve talked about this in previous posts, but in 1929, Edwin Hubble determined from years of observations, that the universe was expanding. Looking out from Earth, every single galaxy that isn’t gravitational bound to our home cluster (Virgo) is receding away from us at a velocity proportional to its distance from us. This has enormous implications on our understanding of the large scale structure of the universe and its origins.
Why is the universe expanding and why is it doing so at an ever quickening pace? Observations by the hubble space telescope tell us that early on in the life of the universe, galaxies were receding much slower than they are now. Dark energy is our explanation for this expansion. In fact, dark energy accounts for 70% of the total energy density of the universe! This is one of the most puzzling topics in cosmology today and many bright minds are trying to figure it out. Needless to say, there are several different hypotheses about the properties of dark energy.
Could it be a property of space? None other than Albert Einstein discovered that it is possible for more space to pop into existence. Further, his theory of gravity proposed that empty space has its own energy. As this energy is a property of space itself and would not be minimized by expansion as more space pops into existence. Thus, as more and more space was created, more energy would come into existence. Unfortunately, this explanation for dark energy falls flat. Its predictions for the total energy of space itself come out to be 120 orders of magnitude off. Bummer!
One fledgling hypotheses states that dark energy is a type of energy fluid or field that fills all of space, a quintessence. I don’t really know anything about this one, but it doesn’t seem likely. Who knows though?
Could Einstein’s general relativity be wrong? This does not seem likely either, every year we discover more and more observational evidence that general relativity is correct. In fact, on May 27, 2011, NASA’s gravity probe-b verified the geodetic effect, or the amount that space is warped by Earth and the frame dragging effect where space and time are pulled by the rotation of the Earth. These are two effects that were predicted by general relativity. As observations like these continue to be recorded, it becomes less likely that we will toss out relativity.
In the end, we just aren’t sure what dark energy might be. You can rest assured that some very smart people are working on this, though J
Dark Flow
This is my favorite “dark” topic. Galaxies in the Centaurus supercluster are moving away in accordance with Hubble flow, but there are variations in their velocities which indicate that another force is acting on them, causing them to recede even faster. Whatever is attracting these galaxies is called the great attractor and it is gravitationally equivalent to several tens of thousands of Milky Ways. Unlike the standard expansion of the universe, this velocity is constant. The matter in our observable universe cannot produce such a gravitational force.
The scientists who proposed this anomaly believe that the culprit could be something which resides outside of the observable universe. Several ideas have been put forth, but the most intriguing to me is that this dark flow could be the imprint of an alternate universe on our own. Plausible? Maybe. Awesome? Yes!
As always, questions are welcome. I love talking about this stuff.
In a shameless show of self-promotion, I'll pos rep anyone who asks a question.
