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The following is a work in progress posted for expert review
Dark Matter Day Q&A
©Bob Field 2018
What is dark matter?
No one knows because unlike ordinary matter, you cannot see dark matter.
How dark is it?
It’s not really dark like black, which absorbs light. It is really transparent or clear as in light passes through it.
Is it invisible?
Yes, but the word invisible means not visible and dark matter is not only invisible in the visible range, but at all wavelengths in the electromagnetic spectrum. In other words it does not interact with light or any other radiant energy electromagnetically – it is electrically neutral and it is does not emit, absorb, or scatter light like ordinary matter. It is transparent like a clear gas or a window, but even more so.
So it does not interact at all with radiant energy?
No. It does, but not electromagnetically. It does bend light just like ordinary matter bends light because light is influenced by the fact that gravitational masses curve empty space and light follows this curvature. It allows scientists to see what is behind a star when the light passes very close to the surface of the star. Einstein’s prediction was first observed more than a century ago.
If it is unseen and unseeable, why does dark matter matter?
Because ... without dark matter there would be no gray matter – that is we would not be here to discuss it. There would be no stars or planets either.
Really, how can it be that important?
Because the standard models of the formation of the universe tell us that the matter in the universe was very uniformly distributed moments after the Big Bang and became even more uniform over the first 50,000 years as radiant energy smoothed out whatever lumps that randomly formed.
Since dark matter does not interact with radiant energy, it did not get smoothed out – it preserved the memory of the primordial density fluctuations that matter “forgot”. It is these density variations that ultimately formed the lumps that we call galaxies, stars, planets, and people. So no dark matter implies no gray matter.
How do you know the standard model is right and there isn’t some other explanation?
We don’t. That’s one more reason why scientists have spent decades looking for dark matter.
How can they look for something if they don’t know what it is?
There are many candidates for dark matter like WIMPs, MACHOS, axions, etc. which have in the past thought to be consistent with the standard model, but over time, knowledge of the observable universe has narrowed down the possibilities while observations have failed to detect other possibilities, so the choices are gradually narrowing.
What if the laws of physics are not what we think they are?
That is a possibility too, but recent observations have made that less likely.
How much dark matter is there in the universe?
The total mass of dark matter in the universe far exceeds the mass of ordinary matter, about five to one. Always has and always will.
Is dark matter distributed uniformly throughout the universe? How is that possible if it has all these density variations, all these lumps?
No. Dark matter is far from uniform. Maybe you are thinking of dark energy, which is believed to be have uniform density throughout the universe. The average densities of dark matter, ordinary matter, radiant energy, and dark energy over cosmological distances are spatially constant, but change over time as space expands. So dark matter is uniformly distributed throughout the universe despite enormous local variations.
How much does the relative abundance of dark matter vary?
Locally here on planet Earth or even in our solar system, almost all matter is ordinary matter. But even in our galaxy, dark matter may have ten times the mass of ordinary matter. And all of the hundreds of billions of stars in our galaxy are made out ordinary matter.
What else does dark matter do?
It holds things together. Most matter is dark matter and its gravitational attraction dominates throughout the universe except in very local patches like stars and planets. Throughout most of the history of the universe, it played the crucial role of slowing the expansion of the universe.
At the end of the cosmic inflation period, the universe was flat, isotropic, and homogeneous with only small density and temperature fluctuations. By the time of recombination, energy flow smoothed out nearly all of the primordial density fluctuations in ordinary baryonic matter.
Non-interactive dark matter retained the primordial density fluctuations, which grew over time as space expanded, giving rise to large scale structure. Gravitational instability formed a web of dark matter halos, filaments, and voids. Dark matter structure attracted baryonic matter which then formed dense structures by radiating excess heat away.
If it does such a great job of holding things together, why is the universe expanding at an ever increasing rate?
Over time, the expansion of the universe has diluted the density of matter and ordinary radiant energy so that their influence has diminished on the cosmological scale of the entire universe. For billions of years now the density of dark energy has exceeded the density of everything else and its ability to push space apart has dominated. In fact, it has helped dilute the average density of the universe due to everything else, matter and radiant energy, so that it went from insignificant for billions of years to being twice as dense as everything else on average.
Is dark energy pulling everything apart?
No, not everything. Locally, where matter is very dense, dark energy is still very low density and does not have any significant effect, so our planet, stars, and local galaxies are not expanding or being pulled apart by dark energy. Since the universe is similar everywhere, that means that stars and galaxies everywhere are also being held together by gravity, not pulled apart by dark energy. They are just getting farther apart.
So what does the universe look like, is it a bunch of galaxies spread out or is there a larger pattern?
Galaxies are important, but most matter is not in galaxies. Most ordinary matter is in the vast empty spaces between galaxies and most dark matter is also not in galaxies. Cosmologists treat the universe as isotropic and homogeneous meaning that it is basically the same everywhere and is very smooth on the largest scale.
Under close examination, it turns out that the largest scale structures in the universe are not galaxies. Most matter in the universe appears to be arranged in the shape of a massive and voluminous cosmic web, a network of nodes connected by filaments enclosing vast voids.
How much dark matter and how much ordinary matter is in each type of large structure?
Very high resolution Illustris computer simulations are providing accurate numerical models of the large scale structure observed in the universe. Halos occupy only about 0.16% of the volume of the universe, but contain about 46% of the dark matter. Halos only contain 23% of the baryonic matter because supermassive black holes that form active galactic nuclei ejected half of the baryons from the halos to the voids. Since the universe has about five times more dark matter than baryonic matter, halos have approximately ten times more dark matter than baryonic matter.
Voids contain about 6% of the dark matter mass of the universe and 30% of the baryonic matter, which means that voids contain about equal amounts of dark matter and baryonic matter. Voids occupy nearly 78% of the volume of the universe and presumably contain 78% of the dark energy.
Filaments are less dense than halos, but denser than voids. Filaments contain about half of the dark matter and baryonic matter while occupying about 22% of the universe by volume. Filaments include very wide structures called walls or sheets.
Since ordinary matter forms stable clumps by dissipating energy and entropy, does dark matter also dissipate energy and entropy to form structures?
Very few sources address this question. Dark matter particles accelerate in a gravitational instability, but there is no obvious way for them to decelerate when they accumulate, so you would expect them to fly past each other rather than form a stable condensed structure.
There are several ways to form structure. First is to preserve the primordial density fluctuations of the early universe. In this case, structure may evolve in an expanding universe without changes in energy or entropy. Second perhaps there is a way for dark matter to dissipate energy and entropy, either directly to space or indirectly through gravitational interactions with ordinary matter that can dissipate energy and entropy.
Where does dissipated energy and entropy go if large scale structure occupies the entire observable universe?
That is a good question just like the question of whether dark energy is conserved since dark energy has constant density and expanding volume. The answer is beyond the scope of this essay, but may be tied to the expansion of the universe.
Is a halo in the simulation similar to a galaxy cluster or what? Are halos located in filaments and voids or only in nodes of the observable cosmic web?
The Illustris computer simulation model treats any region with a density greater than 15 times the average density of the universe as a halo, so to the extent that the model can resolve high density regions, halos could be anywhere in the web.
Do we know anything about the properties of dark matter besides the fact that it has mass and apparently only interacts through the force of gravity?
Yes. Scientists believe that dark matter is mostly “cold”, not “hot” or “warm”. These words are in quotes because in this context, scientists are really describing the velocity of particles rather than the actual temperature of a collection of particles in a solid, liquid, or gas.
How cold is cold dark matter?
By cold, scientists mean it does not move at the speed of light like neutrinos or photons. In the sense of temperature, it may not be what we consider cold in the everyday sense. But then again every day on Earth is different than in space. Most ordinary matter – often called baryonic matter – is as hot as the core of stars, typically ten million degrees Kelvin or more.
I thought space was cold. How can most matter be so hot when the cosmic microwave background (CMB) is a very cold 2.725 K?
The CMB is energy not matter. Photons don’t have a temperature in the sense that matter has a temperature. Temperature measures the random motion of particles of matter whereas photons are always moving at the speed of light. The only sense that photons can be considered cold is when they are radiated by cold matter and therefore have long wavelengths in their blackbody spectrum.
These photons have been traveling through space for nearly 13.8 billion years. When the universe became transparent at the time of recombination, they were very “hot” in the sense of being very energetic, like gamma rays. The number of photons has not changed but as the universe expanded, their wavelengths stretched. Longer wavelengths correspond to lower frequency waves and lower energy per photon.
When the universe became transparent, that is after recombination, matter decoupled from radiation and followed a different path in terms of temperature. When matter condenses due to a gravitational instability, the resulting in-fall increases the kinetic energy. When matter coalescences into stars and planets, collisions between particles convert their kinetic energy into thermal energy, which is nothing more than random motion. Hot matter cools by radiating energy.
Planets in our solar system formed billions of years ago and are still cooling even though the surface of our planet is warming due to solar energy trapped by greenhouse gases. Stars like our Sun on the other hand have cores that are so hot and dense that thermonuclear fusion converts matter to energy during nucleosynthesis. Half of the mass of a star may be in its core at temperatures greater than ten million degrees.
Is most of the matter in the universe hot or cold?
Most of the mass of stars is hot and most of the mass of matter in galaxies is in the stars. But most of the mass – 80% to 95% – in a galaxy cluster is in the intracluster medium or ICM between galaxies. It is very dilute, that is very low density.
So is the intracluster medium hot or cold?
The ICM is a dilute plasma meaning that it is hot, between ten and one hundred million Kelvin degrees. Why? Because it originated in the hot active nuclei of galaxies and it is not dense enough to interact in order to radiate the heat away, unlike dense matter like stars and planets, or even the cool relatively dense molecular clouds in the interstellar medium or ISM in galaxies.
How does the interstellar medium differ from the intracluster medium?
Intracluster is within a galaxy cluster but not within any individual galaxy in the cluster. Interstellar medium is between the stars within a galaxy not between galaxies. Most ordinary matter in a galaxy is in the hot stars. Perhaps 10% of ordinary matter is in the ISM. What is the composition and temperature of the ISM? The composition is fairly complex consisting of several different states of matter. Most authors do not clearly specify how much is in each state.
So does dark matter have a temperature even though it does not absorb or radiate photons? And if so, what is its temperature or is that another unexplained problem?
It seems like every answer leads to more questions and researchers focus on solving specific problems to try to piece the picture together. Astrophysics teachers have so many remarkable concepts and exotic objects to discuss that textbooks cannot address every question that you can think of. We would like to think that some scientists know the answers to these questions, but the answers are not widely disseminated.
How about a yes or no answer - does dark matter have a temperature?
It is widely assumed that dark matter consists of particles moving rapidly in random directions so in that sense it contains thermal energy, but since we cannot measure its temperature in a conventional sense and it does not radiate heat away, we can probably calculate a particle velocity distribution and specify a corresponding temperature, but we cannot use the temperature to predict behavior in a conventional sense.
So which formed first, stars, galaxies, or cosmic webs?
Evidence suggests that the universe formed from the bottom up rather than the top down. In other words some stars formed very early and smaller galaxies merged to form larger galaxies which formed massive clusters and superclusters. But the top down theory has its advocates.
If the total amount of dark matter is constant, what was its density in the early universe?
Today the density of everything in the universe is about 10-26 kg/m3. Dark energy is about 70% of this and matter is about 30% since radiant energy is very small. Dark matter is about 80% of all matter or 25% of the universe. So the density of dark matter is about 0.25 x 10-26 kg/m3 and the density of ordinary matter is less than 0.05 x 10-26 kg/m3. The density of radiant energy is very small because each photon has less energy over time and occupies a much smaller volume.
At the time of recombination, the cosmic scale factor which measures the size of the observable universe was about 1100 times smaller corresponding to a red shift of about 1100. The volume of the universe was 1100 cubed times smaller, or 1.3 billion times denser. So the density of dark matter was about 0.33 x 10-17 kg/m3 when the universe was about 380,000 years old at recombination.
Radiant energy decreases as the fourth power of the cosmic scale factor because each photon occupies a larger volume and has lower energy because of its stretched wavelength. Radiant energy was nearly 1.5 trillion times denser than today. Since the density of dark energy is constant, it did not change, but it was 1.3 billion times less influential when the volume of the universe was that much smaller.
What does dark matter do for the Milky Way galaxy today?
It holds it together. Many stars are moving through space so fast that they would leave the galaxy if it weren’t for the extra gravitational mass provided by dark matter. In fact the determination of star speeds orbiting the center of the galaxy was the clue that there were vast amount of unseen matter. All of the stars in our galaxy seem to have the same speed in orbit based on observations of red shifts. Gravity falls off as the inverse square of distance, so stars farther from the center should be moving slow enough to stay in orbit, just like planets in our solar system move slower if they are located further from the Sun. The conclusion is that most of the mass of the galaxy is unseen and is distributed in a “halo”.
How much dark matter is in a typical large galaxy like the Milky Way?
The mass of dark matter in the Milky Way is approximately one trillion solar masses or 2 x 1042 kg since the mass of the Sun is approximately 2 x 1030 kg. The mass of ordinary matter is about ten times smaller, 100 billion solar masses or 2 x 1041 kg. Most of this is in the 300 billion stars in the galaxy. The interstellar medium is only one tenth of this or 10 billion solar masses. The amount of radiant energy and dark energy is relatively small in a galaxy. So dark matter is about 90% of the mass of our galaxy.
Does dark matter affect whether a galaxy is a spiral, elliptical, or irregular galaxy?
Not so much. Dark matter causes galaxies to form, but does not dictate their shape or evolution.
How did matter become the dominant influence in a universe that was dominated by energy?
The densities of energy and matter decreased as the volume of the expanding universe increased. The total amount of matter did not change, just the density. On the other hand, not only did the density of radiant energy decrease, but the energy per photon and neutrino decreased due to the Doppler shift as space itself expanded and stretched the energy wavelengths. Consequently, the total amount of radiant energy decreased over time, not just its density.
Radiant energy was the dominant influence in the universe for 50,000 years. Eventually its density decreased so much more than matter that matter began to be the dominant influence in the universe. But at this time, the density fluctuations in the universe were so small that matter was too diffuse to condense into stars.
What is Dark Matter Day and why is it on Halloween?
It is a day for science education that has been held every year since 2017 on Halloween, the day we celebrate and hunt for things unseen.