Last night, Dr Natarajan's lecture was titled "Unveiling the Dark Side of the Universe". The lecture centered on the search for dark matter. She began her lecture by stating that, while there has been much progress, the problem of dark matter remains unsolved. Since the Classical period, there have been various conceptions concerning the nature of outer space, with the ancient Greeks proposing the existence of Ether as an extraterrestrial element. Other ultramundane "elements" such as phlogiston and miasma were postulated to explain natural phenomena that were poorly understood. While these conceptions of materials out of the ordinary have fallen by the wayside, the universe remains quite peculiar.
The "ordinary" matter in the universe is really quite extraordinary- it composes less than 5% of the makeup of the universe. Approximately 23% of the universe is composed of dark matter, while 72% is composed of dark energy. Dr Natarajan mused that exploring the nature of dark matter and dark energy can be likened to a crime scene investigation in which there are many clues, but there's no body. There are independent lines of compelling evidence, but the dark matter cannot be directly observed.
Dr Natarajan proceeded to give us a quick overview of current cosmology. While there is no direct data for the Big Bang, there is a "signature" of a period approximately three minutes after the Big Bang, and evidenct of the condition of the universe 400,000 years after the event, which occurred approximately 13.8 billion years ago. The universe started off as a "soup" of dark matter, much of it in clumps. The first galaxies appeared approximately one billion years after the Big Bang. Galaxies developed around clumps of dark matter.
Dr Natarajan then gave us an overview of dark matter and dark energy. She quipped that, when cosmologists don't know the nature of something, they add the adjective "dark" to a term. She reiterated that dark matter composes 90% of matter, while dark energy composes over 71% of the "stuff" in the universe. Cosmologists do not know what dark energy is, but they know what it does- dark energy plays a role in the increase in the rate of the expansion of the universe. In 1929, Edwin Hubble discovered that the universe was expanding... in the 1990's it became evident that the "gas pedal" of the universe's expansion had been floored. In the early stages of the universe, the radiation resulting from the Big Bang dominated the universe, but dark energy is currently the dominant force at work in the universe.
Looking out into space is looking back in time- the speed of light and the age of the universe are both finite, therefore our view of the universe is finite. The farther a particular celestial body is, the further back in time the light from that body originated (the light from the sun takes eight minutes to reach the earth, so our view of the sun is of an eight-minute old Sol). Basically, our observations of distant objects are old "snapshots". There's a boundary beyond which the universe cannot be observed. "Ordinary" atoms are extraordinary- the most common of elements are hydrogen and helium. Heavier elements are produced in starts through fusion reactions, but as the universe cools, the formation of heavier elements becomes more difficult. Dr Natarajan quipped that, while romantics muse about our being made of stardust, one could just as easily say that we are made of cosmic trash. Thanks
The lecture continued with a discussion of gravity, the force of attraction between different bodies which, as formidable a force as it is, falls off over distance. The gravitational forces in the solar system are dominated by the sun- and bodies further from the sun orbit more slowly. Observers tried to apply the same model of rotational velocity to galaxies, expecting them to behave similarly to the solar system, but the galactic rotational curves differed markedly. Astronomers Vera Rubin and Kent Ford observed that the actual rotational curves did not follow the theoretical models and noted the peculiarities without making any claims. The observed rotational curves suggested a lot of mass far from the centers of the observed galaxies and a deficit of mass in the galactic center. The only explanation for these rotational curves is a huge mass of unseen matter at the fringes of these galaxies. Every galaxy has an exterior "halo" of dark matter extending ten times further than the observable matter.
By the 1980s, a theoretical framework was developed to align with the observations. Dark matter does not seem to interact with baryonic matter or photons. It does not emit anything... it's invisible. Dark matter can only be "observed" by its effect on the motion of "nearby" objects.
One particularly interesting line of evidence for dark matter is its effect on light- gravitational lensing. The universe, space/time, can be likened to a sheet (although there is nothing above or below the "sheet") which is bent by gravity. Light traveling through space/time is bent by mass. The more massive a object is, the bigger the "pothole" that it creates in space/time. British astrophysicist Arthur Eddington was able to verify the curvature of space and bending of light while observing the apparent position of a star during a solar eclipse. Light from distant galaxies is bent by dark matter. The shapes of the galaxies that we see are distorted. Some regions of the cosmos are more "cluttered" with mass than others. Observers are able to compare the distribtion of shapes from regions of more mass with those from regions of less mass in order to figure out their shapes and to infer the amount of matter deflecting light. Mathematical models can be used to "undo" the distortion. Dr Natarajan likened this to tomography- images are analyzed "slice by slice" in order to figure out the size of the "pothole" in space/time to determine the amount of mass.
Large conglomerations of dark matter act as good gravitational lenses, and observations of the lensing effects indicate that galaxy clusters are held together by dark matter. Big enough gravitational lenses can split images, but each galaxy has its unique "fingerprint"- its spectrum. Light beams can become split multiple times, helping in the mathematical modeling of dark matter lenses- the distortion of light is systematic, and the position of distorted images allow the modeling of the dark matter making up a particular gravitational lens. The stars themselves only have 10% of the mass needed for distortions- they are small beacons in a much larger mass.
Gravitational lensing was first observed in 1979 by astronomers D. Walsh, R. F. Carswell, and R. J. Weymann who observed twin images of a single quasar- the presence of an intervening galaxy creates a double image.
Observation of the distorting effects of dark matter "halos" allows inferences to be made about their shapes. These halos tend to be smooth and ellipsoid around galaxy clusters, but have an overlay of more granular, or lumpy accretions. The amount of lumps in a particular halo of dark matter leads to a lot of distortion.
The next part of the lecture was accompanied by some particularly gorgeous images. She compared an image of the galaxy cluster Cl2244 obtained in the 80s using the Canada–France–Hawaii Telescope with another image taken a decade later using the Hubble Space Telescope. The finer resolution of the Hubble-obtained image allows for a better "map" of the dark matter causing the gravitational lensing. In the image embedded below, the "arcs" are actually distorted images of galaxies in the cluster:
Dr Natarajan then displayed an image of the Abell 2218 Galaxy Cluster, which she jokingly referred to as the "Brad Pitt and Angelina Jolie of galaxy clusters" due to the frequency with which it has been photographed. Abell 2218 is the first cluster that allowed the mapping of "lumps" of dark matter. In the "undistorted" image of Abell 2218, the younger galaxies appear blue while the yellow "blobs" belong to the cluster itself:
Oddly enough, there don't seem to be any good images of the dark matter "map" of this particular cluster on the t00bz... At any rate, the most dramatic distortion of images indicates tightly packed accretions of dark matter- the "punctuated" fabric of space/time.
Another important feature of gravitational lenses is the fact that they magnify distant galaxies and bring them into view. By "calibrating" the gravitational lens of galaxy cluster Abell 1689, one of the most massive clusters known, astronomers were able to discover the most distant (therefore "oldest") galaxy known. Dr Natarajan likened the cluster to the "optometrist of the cosmos".
Dr Natarajan then paused to ask, "Why are we mapping dark matter?" She indicated that information about the nature of dark matter is sitting in the shape of the dark matter "granules". In a typical galaxy cluster, stars are one percent of the total mass, with hot gas making up another nine percent, and the remaining ninety percent being dark matter. She then showed a breathtakingly beautiful image of the Bullet Cluster, showing the collision of two galaxy clusters:
In the image, the blue sections represent the map of the associated dark matter, while the pink section depicts the energy producing collision of the gas associated with the two clusters. Note the the dark matter doesn't interact with the gaseous collision. It can be inferred that dark matter is not a gas- in the equation of state, P=0 for dark matter.
In the current theoretical model, to which the reconstruction of gravitational lenses corresponds, dark matter serves as the "scaffolding" of galaxy structures. Simulation of the filamentary structure has been undertaken to determine some of the properties of dark matter- cold dark matter would result in granular filaments while warm dark matter would result in smoother filaments. The Cold Dark Matter model seems to better reflect current observations.
The Hubble Frontier Fields project will involve "staring" at a particular cluster "lens" for months to enable the reconstruction of dark matter distribution in order to map filaments of dark matter.
When asked if she had any theories about the nature of dark matter, Dr Natarajan stated that, while nothing is known, one possibility is that dark matter is composed of WIMPs, Weakly Interacting Massive Particles, specifically the hypothetical particles dubbed neutralinos. Columbia University's XENON project is an attempt to detect dark matter.
The inaugural lecture of the Secret Science Club North was fantastic. Dr Natarajan was an engaging, informative speaker who was able to make complex concepts comprehensible, and her enthusiasm for the subject was infectious. Hats off to my good friends Dorian and Margaret for pulling off yet another major success, and thanks to Dr Natarajan for knocking it out of the park. After the lecture, the good doctor was hanging out and entertaining questions from attendees. From what I've heard, the lecture was recorded, so I'll let you guess which of the questioners was yours truly when it is made available. In the meantime, here is a short video of Dr Natarajan talking SCIENCE:
I drank considerably less last night than I do at the Bell House. To replicate the Secret Science Club North experience, you can simply sip one drink while watching rather than chugging a sixer.