Puzzling Over Primordial Lithium
If someone were to ask you about the importance of lithium, perhaps the first thing you
might think of is the battery keeping your mobile phone connected to the world. Or,
perhaps you’d think of that little pill that makes your grandmother feel better about her
day. However, lithium, which sits as the third element in the periodic table, has much larger
implications for our universe.
Lithium is regarded by astronomers as one of the most interesting, yet most puzzling,
elements present in our universe. It is among the first elements to be synthesized just after
the big bang. It is also produced in stars during the nuclear burning of hydrogen into
helium. This single light element can provide important constraints for both the birth of our
universe as well as for models of the evolution of stars. However, models which serve to
predict the abundance of lithium in the universe and measurements of lithium in stars, are
afflicted by a single problem. They do not agree.
The cosmic lithium problem
In current cosmological models, large-scale structure grows hierarchically in a cold dark
matter dominated universe, born from the big bang. The models assume that the energy
density of the universe is also dominated by dark energy, which appears to behave as a
‘cosmological constant’ and explains the accelerating expansion of the universe. The success
of this model comes from its exquisite agreement with observations of large scale structure
and the cosmic microwave background (CMB).
The CMB represents all of the left-over radiation during the formation processes that took
place just after the big bang. In 2001, the Wilkinson Microwave Anisotropy Probe (WMAP)
was launched into space and has since then been taking high precision measurements of the
temperature fluctuations across the CMB. The structure measured by WMAP has shown
markedly similar results to those predicted by the cosmological models. Further, in
conjunction with the WMAP data, cosmologists can also precisely predict the nuclear
processes that took place to create the first elements in the universe.
Just a few minutes after the Big Bang, the nuclei of hydrogen in the form of deuterium,
helium in the form of two isotopes, 3-helium and 4-helium, and that of the lithium isotope
7-lithium were synthesized. Big Bang nucleosynthesis (BBN) theory serves as an extension
of current cosmological models, which predicts the reactions that took place, and how much
of each element were synthesized in the early universe. The key parameter in the theories,
which determines the amount of the primordial elements produced, is the relative amount
of baryons (nuclei-forming particles) and radiation.
One of the several robust measurements that comes from WMAP is the baryon-to- photon
ratio, precisely the input needed for BBN theoretical models. Combining this information
with the BBN models, astronomers have predicted the amount of the primordial elements
that were produced just after the big bang. In order to test these
predictions, astronomical objects, which are comprised of the primordial elements, must be
identified.
Fortunately, the primordial elements have been detected in astronomical phenomena in the
relatively nearby universe. For example, deuterium is measured in giant gas clouds in
distant galaxies, while helium can be studied in large gas regions of nearby galaxies. Both
measurements have shown excellent agreement with the predictions from big bang
nucleosynthesis.
Lithium, on the other hand, is primarily measured in stars. Astronomers have long assumed
that the primordial abundance of lithium is preserved in the atmospheres of the oldest stars
in our Milky Way Galaxy. When a star forms, the chemical composition of its outer
atmosphere should reflect that of the chemical content of its surroundings. There are,
however, nuclear reactions taking place, in which lithium is produced during hydrogen
burning, in the cores of the stars. Lithium is, however, an extremely fragile element. It is
readily destroyed at temperatures exceeding a few million degrees Kelvin. Therefore, any
lithium produced in the core of a star is immediately burned, creating helium.
Since the lithium produced within a star is quickly destroyed, the lithium abundance in the
atmospheres of the oldest stars should only reflect that of the primordial abundance of
lithium. The star must be old, because a relatively young star would also receive additional
contributions of chemical elements from previous generations of stars that have polluted
the primordial abundances with those synthesized during the nuclear reactions taking
place inside the stars themselves.
Further, cool stars like our Sun are affected by a process called convection. Convection
circulates material from the outer atmosphere of a star deep into the the hot interior where
nuclear burning is taking place. Hence, any lithium in the outer atmosphere would undergo
constant destruction as it is circulated to the interior of the star.
The oldest stars, which contain only light elements in their outer atmospheres (otherwise
known as metal-poor stars), however, have very thin convection zones, which do not extend
very deep into the interior. It was therefore believed that the lithium would not be
circulated to regions with temperatures hot enough for it to burn. And so, the primordial
abundance of lithium is preserved.
In 1982 two French astronomers, Monique and François Spite, measured the lithium
abundance for several metal-poor stars, finding that the lithium abundance was constant
amongst the stars. This threshold in the lithium abundance, thought to be the primordial
abundance of the universe, was later called the ‘Spite Plateau’. Subsequently, there have
been many studies to measure the abundance of lithium in metal-poor stars both in the
stellar halo of the Milky Way Galaxy as well as in metal- poor stars found in dwarf galaxies
that surround the Milky Way. Measurements have shown an incredible constancy in the
lithium abundance of these stars over a range of
stellar effective temperatures and gravities, further enforcing the idea of the Spite Plateau.
The lithium abundance at which the Spite Plateau lies is about three to four times smaller
than what has been predicted by standard big bang nucleosynthesis theory. This constitutes
what is known as the ‘cosmic lithium problem’. This large discrepancy with the predicted
primordial value of 7-lithium has stimulated much interest in both the destruction and
production of lithium in the interiors of stars, as well as, in possible extensions of the
Standard Model of particle physics.
Where has all the primordial lithium gone? Presently, there are two possible solutions.
Either the primordial lithium was depleted in the universe before the formation of the old,
metal-poor stars, or the stars themselves are depleting the lithium by some other
mechanism than was previously thought.
Dark Matter
How might the primordial abundance of lithium have been depleted in the early universe,
before the oldest stars formed? The answer could lie with dark matter.
There has been overwhelming evidence that dark matter exists in our universe. However,
we still do not know what it is. A popular theory among particle physicists is that dark
matter is comprised of weakly interacting massive particles (WIMPs). These WIMPs are the
stable products of the decay and transformation of more massive particles during big bang
nucleosynthesis. Big bang nucleosynthesis, however, sets very tight constraints on the
amount of baryonic matter in the universe. The vast majority of dark matter should then be
non-baryonic and therefore demands physics beyond that of the Standard Model of particle
physics.
One such theory that proposes more exotic physics is supersymmetry (or SUSY). In nature
there are two types of particles: bosons and fermions, which differ in their intrinsic angular
momentum, a property physicists call ‘spin’. SUSY relates each of these two types of
particles with supersymmetric partners or ‘superpartners’ having the other type of spin. In
such a scenario, the lightest of these superpartners is expected to be electrically neutral and
stable, and would have only gravitational interactions. This lightest superpartner can be
considered a candidate for dark matter. Further, the decays of unstable and heavier
superpartners could produce Standard Model particles that would interact with their
surroundings. It is these interactions that could lead to the depletion of the primordial
lithium.
Another theory that has been gaining interest in the community is that dark matter is made
up of particles called axions. Axions do not destroy excess lithium (as is proposed for the
SUSY theories). Instead, their influence is one of causing the CMB to be slightly darker than
it would have otherwise been, due to a form of axion-induced photon ‘cooling’. Thus, we
would underestimate photon energies during the BBN, thereby causing us to overestimate
the resulting lithium abundances. So, in this case, a dark matter particle is not actually
destroying lithium. Rather, it is causing us to make an error in our BBN calculations.
Both SUSY and axions are very elegant concepts, which, if they were to be realized in
nature, could explain both the properties of dark matter as well as the destruction of
lithium before the formation of the oldest stars. SUSY, however has recently come under
experimental scrutiny. The ATLAS and CMS experiments at the Large Hadron Collider (the
LHC) at CERN in Geneva, Switzerland, have been searching for evidence of SUSY and were
expected to have detected the production and decay of some of these superpartners in their
current operations. However, no direct evidence of SUSY has been found thus far. Should
this situation persist when the LHC operates at its highest energy and intensity later in the
decade, then this would present a serious challenge to the concept of supersymmetry in its
current formulation. Similarly, axions have not yet been observed experimentally. There
may be a chance to detect one with the Axion Dark Matter Experiment (ADMX), located at
the Center for Experimental Physics and Astrophysics at the University of Washington,
which is due to begin at the end of the year. It may also be possible to have an indirect
detection at the LHC. So there is marked interest in following the progress of these
experiments in both the particle physics and astrophysics communities.
Stellar Depletion
Astronomers have taken a different approach to tackle the lithium problem. The problem
may not lie with the initial abundance of primordial lithium, but with our understanding of
the processes that take place inside of stars.
Recently, astronomers have measured the lithium abundance of the metal-poor interstellar
gas in the Small Magellanic Cloud. The advantage of using metal-poor gas is that the
abundances are unaffected by processes that would otherwise alter the chemical
composition of a star. Measurements in the gas are extremely difficult, however, since they
require an accurate measure of the ionization fraction of lithium as well as the amount of
lithium contained within dust grains. Current results have shown that the present-day
abundance of lithium in the interstellar gas of the Small Magellanic Cloud matches that of
BBN predictions, thereby adding to the puzzle. This suggests that the interstellar gas has
not been enriched with lithium produced during stellar nucleosynthesis, which supports
the claim that lithium-depletion may be taking place in stars.
In order to determine the amount of an element in a star’s atmosphere, astronomers
compare the observed absorption line strengths of a given element in the spectrum of a star
to stellar model atmospheres, which contain predictions for the abundance of that element
for a given set of atmospheric parameters. These models, however, are severely limited by
the physical assumptions imposed on them. For example, the majority of models assume
that the atmosphere of a star is well-represented by a one dimensional model which
includes a static temperature and pressure structure in local thermodynamic equilibrium.
Further, the magnetic field of a star is ignored entirely,
which could have effects on convection processes in the star. These assumptions were
primarily prescribed due to computational limitations in the past. However, even though
new models are now being created, which relax these assumptions, the majority of
astronomers in the field still use the old prescriptions.
These models are what led astronomers to initially think that there are no processes in
metal-poor stars that can significantly alter the chemical composition on the stellar surface.
However, stellar modelists no longer think this is the case. It is now believed that extra
mixing processes take place within stars that would both dilute the surface composition
with chemical material that could only be produced in the core of a star as well as deplete
those elements on the surface of stars.
Evidence for such phenomena have been plenty. For example, in giant stars, the observed
surface carbon, nitrogen, and oxygen abundances reflect that of which is being produced in
the core of the star. This would suggest that an extra mixing mechanism is transporting
chemically enriched material to the surface of the star. On the other hand, a process called
‘atomic diffusion’, by which material on the surface of a star settles towards the interior due
to gravitational forces, has been observed in the Sun. This process would deplete some of
the surface abundances of the Sun.
More importantly, it is expected that the effects of atomic diffusion would be greater in
metal-poor stars, the exact stars that astronomers assumed contained the primordial
abundances of lithium in their atmospheres. Atomic diffusion would effectively deplete the
primordial abundance of lithium in the atmosphere of a star, and hence, the abundance of
lithium measured in metal-poor stars would not match that predicted by BBN. Recent
studies have further supported lithium-depletion in stars, showing that the most metalpoor stars (those we expect would have abundances of lithium closest to the primordial
value) have even less lithium in their atmospheres than their more metal- rich
counterparts.
Atomic diffusion has not come without problems. Early models, which contained atomic
diffusion failed to produce lithium abundances that matched the observed Spite plateau,
which shows a flat lithium abundance for the majority of warm, metal-poor stars.
Essentially, early formulation of atomic diffusion was too efficient, and the lithium
abundance was predicted to deplete too quickly. Recent models have included additional
effects such as radiative levitation and turbulent mixing, which have alleviated some of the
problem. These effects serve to slow the depletion of lithium due to atomic diffusion.
Present studies have focused on globular clusters to test these extra mixing
parameterizations. Globular clusters can serve as testbeds for stellar evolution. In most
cases, their constituent stars have the same age and chemical composition. Thus, relative
differences in the chemical composition of stars at different stages of evolution in a specific
globular cluster can provide clear indications that the stars have undergone mixing in their
atmospheres. Current results suggest that atomic diffusion (which includes radiative
levitation in addition to gravitational settling) in combination
with extra turbulent mixing can fit quite well to the data. However, in the case of turbulent
mixing, the physical process by which it occurs is still unknown, thereby leaving the
quantity as a free ‘ad hoc’ parameter. Current efforts are now focused on investigating the
physical phenomena (such as gravity waves) in the interior of stars that could account for
this turbulent mixing.
The Bottom Line
Lithium is still a problem. The predicted abundance of primordial lithium in BBN theory is
still almost four times as much as that measured in metal-poor stars. We may, however, be
edging closer to reconciling this significant difference. New theories about dark matter have
proposed that dark matter is comprised of particles that could have either depleted the
primordial abundance of lithium or caused our initial measurements to be overestimated.
On the other hand, the latest astrophysical models have shown that the lithium abundance
on the surface of stars can be depleted by internal mixing mechanisms. These cosmological
and stellar theories, however, have parameterizations that still need to be realized
physically in nature, and so much work is still needed. Perhaps within the next decade we
will have our solution.