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Dark Matter and the Universe

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! Dark Matter and the Universe Topic 4 Hunting for Baryonic Dark Matter Black Holes, Dead Stars, Neutrinos & the Primordial Soup Why is the dark matter not ordinary matter we can not see?! Contents of Topic 4 In this Topic we move to consider the possible Baryonic candidates for dark matter, their nature and the arguments that eventually lead to the conclusion that dark matter can not be composed of hidden Baryons. We cover: " Gas, dust, rocks and smaller material as dark matter " Massive Astronomical Compact Halo Objects (MACHOs), Brown Dwarfs, Dead Stars and Stellar Remnants " Hunting for MACHOs with Gravitational Microlensing " Microlensing theory, Amplification Factor and Einstein Time " Very Massive Objects (VMOs), Black Holes, & Neutron Stars " Big Bang Nucleosynthesis, the CMBR and #B " The death of Baryonic Dark Matter " First ideas on particle dark matter - Antimatter and Neutrinos Baryonic Dark Matter Candidates " It is clear from the evidence shown that there are copious amounts of Missing Matter or Dark Matter in the Universe. " Whatever this material is it must not give off light or interact with light, otherwise we would see it! " A first step to unravelling this huge mystery, investigated by the first scientists involved, is to consider objects or classes of ordinary material, i.e. Baryonic Material, that are known to exist but that might be hidden from us. An example is Hydrogen Gas: " The term Baryon basically refers to protons and neutrons. Strewn throughout the Universe are "trees" of H gas that absorb light from distant objects. For instance they leave absorption lines in distant quasar's spectra. Baryonic Dark Matter Candidates " A more complete list of possible Baryonic Candidates, roughly in order of size, is as follows: (1)! Gas - hot and cold gas, hydrogen and helium (2)! Snowballs - particles of frozen gas (3)! Dust - particles that include heavier elements like Si (4)! Rocks and Small Planets - including asteroid size objects (5)! Dim Stars - including Brown Dwarfs and Black Dwarfs (6)! Neutron Stars - remnants of supernovae (7)! Black Holes - remnants of bigger supernovae " Candidates labelled (5) have been the subject of intense searches in the halo of our galaxy. They are often called: Massive Astronomical Compact Halo Objects (MACHOs) " Candidates (6) and (7) come under the heading Very Massive Object (VMO) Gas, Dust and Smaller Material " This type of material is actually hard to hide in the Universe. It can produce spectral lines if hot or absorption lines due to background stars. Gas as Baryonic Dark Matter " A first obvious Baryonic Candidate might be objects made of hydrogen or helium gas, the most abundant elements in the Universe. But each class of this has a problem:! (1)!so called “Snowballs” (small lumps of frozen hydrogen) – it turns out that these would be expected to evaporate. (2)!Hot Gas - this is expected to emit x-rays, but very little such radiation is seen. (3)!Cool Neutral Hydrogen Gas – this should absorb background light from distant objects like quasars, but not much of this absorption is seen either.! Dust, Rocks and Small Planets " So what about material with elements more complex than hydrogen, forming dust, rocks, asteroids or even small planets. It certainly exists, here is an example Porous Chondrite interplanetary dust particle. " But if the Universe is filled with a lot of this stuff then stars should be contaminated with significant abundance of “Metals”. This is not seen. We see mainly hydrogen again. " Also rocks and dust can be observed by obscuration of background light. It is actually hard to hide in the Universe. It can produce spectral lines if hot or absorption lines due to background stars. Not enough of this phenomena is seen. None of this can explain the dark matter problem. MACHOs " MACHOs are another possibility, basically Very Low Luminosity Stars, sometimes called Dead Stars or Stellar Remnants, stars known as Brown Dwarfs and Jupiter-like Objects. These objects would have mass < ~0.08 M⦿. " Below this mass they never attain a high enough central temperature for the fusion reactions that convert H into He normal in larger mass stars to start and produce high luminosity. " The only energy they can radiate comes from Gravitational Energy as they slowly contract to a final dense state. There is an initial fairly fast contraction which can cause significant luminosity but it does not last long. Hunting for MACHOs " So MACHOs can not be seen with telescopes but it is possible to use so-called Gravitational Microlensing (GM). This exciting method to search for faint stars in the halo of our galaxy was first proposed by Bohdan Paczynski in 1986. " GM is a variant of the lensing described earlier but here the lens is a MACHO in the halo of our galaxy (a much smaller object than considered before) and the source is a star in a nearby galaxy, usually the Large Magellanic Cloud (LMC). Bohdan MACHO Paczynski Star in nearby galaxy Earth Gravitational Microlensing Theory " We can use the same equations for GM as previously used for Gravitational Lensing. However, in Microlensing there are a few interesting differences to what is happening: (1)!For a small lens like a Brown Dwarf we might expect an Einstein Ring or Multiple Images, but in practice the light bending is too small for this to be resolved in a telescope on Earth. Instead we see a general brightening resulting from the multiple images being superposed on each other. MOVING MACHO (2)!Another vital difference is that STAR IN OBSERVER we expect the MACHO to be LMC ON EARTH moving in the halo such that the alignment between Observer, MACHO and Lens that produces this brightening will only occur for a timescale of ~days to months. Microlensing Example " The image here shows an actual example candidate MACHO Event. We see a sequence of pictures of a small section of a star field taken over a period of a few weeks. Note how one of the stars in the centre gets brighter and then dims. " A plot of the brightness vs. time gives us a characteristic Light Curve for the event as opposite. We see a Transient Brightening of the background star as the MACHO passes by. Microlensing Theory? " We can use the same equations to describe Microlensing as we examined for general lensing previously. r here is distance of the MACHO from the line-of-sight r source observer MACHO (lens) " The MACHO moves across the line-of-sight so r changes with time, i.e. r(t). We can call it the angular separation of the lens from the observer-source line-of-sight vs. time. " The Impact Parameter b, as defined previously, is effectively the instantaneous value of r. The Amplification Factor " The source brightness is amplified by an Amplification Factor A that depends only on how close the alignment, r(t), is between observer, lens, and source. It is given by: here u(t) is defined in terms of r(t), the angular separation of the lens from the observer-source line-of-sight, divided by the Einstein Radius θE and is given by: where t0 is the time at Peak Brightness and u0 = u(t = t0). τE is the Einstein Time, the time taken by the lens to travel the angular distance θE. u = 0 corresponds to perfect alignment. The Amplification Factor ‣ So when t = t0, we have Peak Brightness. ‣ The smaller u becomes, the bigger the amplitude A, and also the smaller the value for u0 = u(t = t0), the bigger is the value that the Amplification Factor A can achieve. ‣ In the extreme case that u0 = 0, which corresponds to a situation where we get perfect alignment of source, lens and observer at t = t0, then A would become infinite. ‣ Here is an example light curve u0 for a point like MACHO for different values of u0 vs. time. ‣ The vertical axis is logarithmic multiplied with 2.5 to obtain the astronomical logarithmic brightness measure Magnitude. How to Identify a MACHO ‣ Microlensing differs from Macrolensing (the Strong and Weak Lensing with galaxies) in that the value of u changes significantly in a short period of time, ~days to months. ‣ Note also that observation of a MACHO Event can allow us to determine a value for the Einstein Time τE, but this is not sufficient to allow the MACHO mass to be determined because we also need to know the distance D and the velocity of the MACHO vM. ‣ In other words the main observable, the Einstein Time, is a degenerate function of the lens mass, distance and velocity. We can not determine them from a single event. ‣ So in practice to get at the masses we need multiple events and estimates of the distances and velocities. How to Identify a MACHO ‣ Another issue is that many stars naturally have variability. It is important to distinguish such stars from MACHO Events. ‣ This can be done because MACHOs have three particular characteristics: (1)! A MACHO Event should never repeat (they are too rare). (2)! The Light Curve for a MACHO should be symmetric in ! time, i.e. rising and falling with the same shape. (3)!The magnitude of the Light Amplification should be the ! ! same in all wavelengths e.g. the blue and red wavebands !as typically measured in astronomical observations. " This difference arises because the change in light for a MACHO concerns purely gravitational effects, whereas in a variable star, such as a Ceyfert Variable, the changes are due to physics processes to do with the star itself. MACHOs Found! or Not " Several extensive searches have been undertaken for MACHOs in the halo of our galaxy by observing many stars in the LMC over several years.
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