Daniel Wolf Savin © 2009

Star Formation in the Early Universe

In the first three minutes of the universe, after the big bang, nuclear fusion formed essentially only the elements hydrogen and helium. This process, known as big bang nucleosynthesis (BBN), generated insignificant amounts of heavier elements. The universe was devoid of stars and galaxies.

Immediately after BBN, the extreme heat of the early universe prevented free electrons from combining with hydrogen and helium nuclei to form neutral atoms. As soon as a nucleus captured an electron, the heat of the universe would immediately rip the electron back off (through collisions either by high energy photons or by energetic electrons). Only after about 400,000 years of expanding did the universe cool down enough to allow nearly all the free electrons and nuclei to combine, thereby forming neutral hydrogen and helium atoms. There were still no stars or galaxies.hublot replica replica watches replica watches uk

As the universe expanded, it cooled. The diminishing heat left over from the big bang radiated light of decreasing energy. Eventually this thermal radiation ceased to have enough energy to interact with matter and the two “decoupled”. This generated the cosmic microwave background (CMB), a snapshot of the universe at about 400,000 years old taken by the light scattering off the matter during that epoch. The 1978 Nobel Prize in Physics recognized the discovery of the CMB. The 1990 Nobel Prize in Physics honored the discovery of both the basic form of the cosmic microwave background radiation as well as the small 1 part in 100,000 variations seen in different directions.

Out of the quantum fluctuations present shortly after the big bang, arose the miniscule graininess observed in the CMB. These small matter density variations seeded the formation of the first stars which would eventually grow out of the primordial mixture of hydrogen and helium. The gravitational attraction from regions of space with slightly above average density drew in matter from less dense regions, forming giant clouds of atomic hydrogen and helium. Out of these clouds, the first stars ultimately formed.

Hydrogen (H) constituted the most abundant and important element in these hot primordial clouds. H consists of a positively charged proton in the nucleus orbited by a negatively charged electron with a total charge of zero. A very small fraction of these H atoms captured a second electron from one of the few remaining free electrons, creating negatively charged hydrogen H-. Subsequently some of the H and H- associated to form molecular hydrogen (H2) with the extra electron detaching. The bulk of the H2 during this epoch in the early universe formed via this associative detachment (AD) reaction.

As the primordial clouds began to accrete material and to merge with other clouds, their density and temperature both went up. These increases accelerate the chemistry occurring in the clouds, causing more H2 to form in their core. When they reached a temperature of ~ 3,000 Kelvin (about half the temperature on the surface of the Sun), H2 became the dominant coolant of the gas. At this point hot H atoms collided with the H2, giving up some of their heat to the H2. The molecular hydrogen in turn radiated this energy out of the cloud in the form of photons. One can think of H2 as the catalytic converter of the early universe allowing the primordial clouds to cool and for the first stars to condense out, sort of like beads of water suspended in a cloud on a foggy day.

Generating reliable computer simulations for the formation of the first stars requires accurate predictions for the H2 abundance in the primordial clouds during this epoch. This in turn necessitates understanding not just the AD mechanism by which the H2 formed, but also quantifying the probability for H and H- forming H2 via the AD reaction. Although this reaction involves the simplest element forming the simplest molecule that exists, surprisingly we still do not accurately know the probability for AD to form H2. Over 40 years of theoretical and experimental studies have failed to converge, demonstrating the continuing difficulty and complexity of attempting to understand even the most basic atomic and molecular processes.

The uncertainty in the AD reaction has major implications for our understanding of the formation of the first stars. It renders uncertain predictions of whether or not a given primordial cloud will form a star. And should the cloud form a star, the most likely mass for the first stars evades reliable prediction. In an attempt to help resolve these cosmological questions, my group has put together a novel experimental apparatus to measure in the laboratory the AD probability for H and H- forming H2.

Laboratory Simulations of Molecular Hydrogen Formation in the Early Universe

The experiment begins with a gas discharge, sort of like a neon sign, where we generate H-, extract it, and accelerate it to a speed able to circle the earth at the equator in less than 3 seconds. This corresponds to a temperature of ~ 200 million Kelvin (about 10 times hotter than the center of the Sun). Collisions readily destroy the tenuously bound H-, requiring us to carry out our experiments under vacuum. A vacuum chamber roughly 10 meters (30 feet) long and 10 centimeters (4 inches) in diameter houses the experiment.

To generate the H beam we shine a 1.4 kilowatt infrared laser beam through the H- and detach electrons from about 10% of the H-. This produces a beam of atomic H embedded within and copropagating with the H- beam.

We then use a trick to mimic the temperatures relevant in the early universe. The two beams speed along together like fast cars on a highway, but they move in the same direction with nearly the same velocity. As a result, like cars on the highway the relative velocity between the beams is very slow, corresponding to temperatures below 3,000 Kelvin. The sound in channel one of the Star Womb Project corresponds to the relative collision energy between the two beams.

While the two beams co-propagate, some of the H- and H undergo AD forming H2. After about 1 meter (3 feet), applied voltages remove the H- beam from the H and H2. The challenge now becomes one of detecting the ~ 100 per second H2 from the billion times more intense H beam. Standard electrical and magnetic methods for manipulating beams fail when it comes to neutral atoms. To solve this dilemma, we employ a novel trick. The beams are sent through a cell of helium gas. Inside the cell ~ 5% of the H and H2 collide with the helium, lose an electron, and become ionized or positively charged H+ and H2+. Utilizing standard electrostatic methods, we can then readily separate the signal H2+ from the neutral H and H2 as well as from the H+.

Extracting the signal H2+ from the various backgrounds produced individually by each beam remains the final issue to discuss. We extract this signal by chopping the two beams so that we alternate between both on, only the H- on, only the H on, and both beams off. These four phases of data collection correspond to the sound channels two through five of Star Womb. We determine the signal essentially by taking the data with both beams on and subtracting from this the data from the other phases.



Though the experiment is performed in a vacuum, the research cannot be carried out in a vacuum. I would especially like to recognize my current and former group members Drs. Hjalmar Bruhns, Holger Kreckel, Michael Lestinksy, and Ken Miller as well as my collaborator Prof. Xavier Urbain from the Université catholique de Louvain in Belgium. All have been central to the success of the research. Financial support for this work comes in part from the National Science Foundation Division of Chemistry and Division of Astronomical Sciences.