BOOMERanG and the Sound of the Big Bang

Two years ago, astrophysicists studying Type Ia supernovas discovered that our universe is a much stranger place than we had imagined, with invisible vacuum energy accelerating its expansion.  (See my column about this in the May-1999 Analog.) However, new astrophysical observations from the BOOMERanG experiment (Balloon  Observations Of Millimetric Extragalactic Radiation and Geomagnetics), a balloon-borne cryogenic microwave telescope measurement that flew at an altitude of  about 24 miles over the Antarctic, indicate that our universe is also rather ordinary, in that its space is maximally flat.  The term “flat” as used here means that the space of our universe is neither positively curved like a ball or negatively curved like a potato chip.  It appears that in our universe, the tendencies of space toward positive curvature from gravitational attraction and towards negative curvature from expansion kinetic energy are precisely in balance, leaving space on the average completely free of curvature.

In general relativity the curvature of space is characterized by the density parameter W_total , which is less than 1 for negative curvature, greater than 1 for positive curvature, and exactly one for flat uncurved space.   The BOOMERanG measurement has shown that W_total = 1.00 ± 0.12.  There are theoretical reasons for taking this as an indication that W_total is precisely 1.

Our present “standard model” view of Big Bang cosmology tells us that about 14 billion years ago the Big Bang started from a tiny point that explosively unfolded to make what became our universe.  In the first few picoseconds of the Big Bang, for reasons that are not understood, the universe expanded very rapidly in a process described as “exponential inflation”.  During this inflationary period, the diameter of the universe increased much faster than the speed of light.  Then the expansion slowed, and the universe passed through phases in which the four forces (strong, weak, electromagnetic, and gravitational) separated, changed strengths, and resolved themselves, in which quarks and leptons formed and acquired mass, in which composite particles including protons and neutrons formed from the quarks, and in which the protons and neutrons combined to form primordial nuclei of deuterium, helium 3 and 4, and lithium 6 and 7.

Finally, about 300,000 years after the initial Big Bang, the universe had cooled enough to allow the positively charged protons and heavier nuclei to combine with the negatively charged electrons to form neutral atoms.  At this point the “primordial soup” of the universe became electrically neutral.  Photons of light in the medium, which up to that time had been trapped by rapid and repeated absorption and scattering with the surrounding charged particles, abruptly found themselves in a transparent medium.  They became free particles, able to travel large distances without interactions.

For the most part, these photons have been traveling through the universe since that time without scattering.  However, the expansion of the universe has stretched their wavelengths by a factor of a thousand.  They reach us now, 14 billion years later, as very low energy photons of the cosmic microwave background.  Their wavelength distribution, originally produced by the very hot plasma of the early universe, is now characteristic of a cold “object” with a temperature that is only 2.73^o above absolute zero.

This cosmic background radiation (CBR) from the Big Bang was first detected in the 1960s.  It has been repeatedly measured with increasing precision for the last four decades, culminating with the COBE satellite measurements in the early 1990s.  However, in the past five years it has been realized that the CBR has encoded within it some definitive information about the characteristics and composition of our universe.  We are only now, through BOOMERanG and similar measurements, gaining the capability of decoding that information.

Theoretical studies have shown that as the early universe expanded, sound waves propagated through the dense medium that closed back on itself, so that the hypersphere of the universe rang like a bell.  The detailed frequency spectrum of the sound waves that permeated the primordial universe, literally the sounds of the Big Bang, depends on details such as the expansion rate, the energy balance, and the matter density of the universe at the early age of 300,000 years.

But how could one possibly listen to sound waves that had dwindled away and died 14 billion years ago?   Fortunately, there is a way.  The primordial sound waves were present at the time that light decoupled from matter to form the CBR.  The sound waves, because they produced regions of compression and rarefaction in the medium, modulated the CMB, with some regions being slightly hotter or colder than other.  This modulation is predicted to still be present in the CBR at a scale of less than 2 degrees in angle.  Basically, to see the effects of the primordial sound waves one must look for variations in the temperature of the CBR of about 1 part in a ten thousand through a peephole that spans a cone in the sky with an opening angle of 2 degrees or less, and scan this peephole across the sky.

Unfortunately, this modulation effect had not been predicted when the COBE measurement was designed, and so that measurement had an angular resolution of about 7 degrees in angle, a resolution that averaged away all of the crucial information.  Therefore, it was realized about five years ago (see my column on the 17th Texas Symposium in the August-95 Analog) that a whole new set of CMB measurements was needed, and that these would place very tight constraints on the parameters of the universe.

BOOMERanG is one of these new measurements.  It is an international scientific collaboration of astrophysics groups based in Rome, London, Pasadena, Berkeley, Florence, Geneva, Santa Barbara, Lisbon, Paris, Amherst, and Toronto.   Because of a combination of careful design and good luck, the BOOMERanG measurements have significantly improved our knowledge of the parameters of the universe.

BOOMERanG consists of a microwave telescope that focuses and concentrates the microwave radiation from a sky-angle of 0.2 degrees on three cryogenic Si₃N₄ web detectors that are sensitive to frequencies of 90 GHz, 150 GHz, and 240 GHz, respectively.  The webs change temperature in response to the energy delivered by the focused microwaves.  The entire device is mounted between shields that blocked the radiation from the Earth and the Sun.  The sensitive aperture of the telescope is scanned over the sky, providing a 3-frequency image of the sky mapped into about 45,000 pixels in each frequency channel.

The telescope was lifted from McMurdo Station in Antarctica to the outer fringes of the atmosphere at an altitude of 120,000 ft by a large 2.8 × 10⁷ ft³ helium-filled balloon.  The instrument remained there, gathering data over a patch of sky with an angular area of 1800 square degrees, for a period of 10.8 days between December 29, 1998 and January 9, 1999.  The balloon followed a circular path centered at the South Pole, and returned to within 80 miles of its launch site.  There the telescope was cut down and retrieved.

The raw data produced by BOOMERanG is a time sequence of signals in each frequency channel consisting of 5.4 × 10⁷ 16-bit samples per channel.  This sequence had to be converted by data analysis, first into a sky map of temperature variations at the parts in 10,000 level, and then by Fourier analysis into a distribution of the spatial frequencies or angular sizes of the observed variations.   It is these distributions that can be compared with models of the early universe.  If the universe if “flat”, in the sense defined above, then the angular size is expected to have a peak at about 1.8 degrees.  This corresponds to a “standing” sound wave in the early universe that has a number of maxima and minima (or “multipolarity”) L in its pattern of compression and rarefaction that is L=197±6 or about 200.

From calculations of the standing sound waves in the early universe, it is found that the value of L is directly related to the flatness or curvature of the universe.  For example, L=125 would correspond to an open universe with negative “potato-chip” curvature and with W_total = 0.66.  On the other hand, L=300 would correspond to an closed universe with positive spherical curvature and with W_total = 1.55.  The observed value of L=200 is precisely what is expected for a flat universe with W_total = 1.0.   Therefore, BOOMERanG has accomplished a remarkable feat.  It has “heard” the sound of a perfectly flat Big Bang from a distance of 14 billion light years!

But there is also another important aspect to the BOOMERanG measurements.  The data on the accelerating expansion of the universe from studies of Type Ia supernovas mentioned above places certain constraints on the parameters describing the universe, the age T₀ of the universe, the total density parameter W_total, the matter density W_M, and the vacuum energy density W_L.  The BOOMERanG measurements place other constraints on the same quantities.  And, plotted on an appropriate diagram, these constraints form an “X”, with the cross point indicating the parameter values that are consistent with both measurements.

These lead to the conclusions that the universe has an age T₀ = 14 billion years, W_total = 1, W_M, = 0.3 and W_L = 0.7.  The surprisingly large value of W_L, which implies the dominance of vacuum energy in over normal mass-energy and dark matter mass-energy in the universe at present, is a result that rocked the astrophysics community two years ago.  This is now supported by the BOOMERanG results, which come at placing constraints on the parameters from a completely different direction.

We now understand that the space of our universe is flat, that it is expanding at an ever-increasing rate, and that it will never re-collapse to a “Big Crunch”.  What we don’t understand is why.  But physicists, astronomers, and astrophysicists around the world are working on that question too.

Electronic reprints of over 100 "The Alternate View" columns by John G. Cramer, previously published in Analog, are available on-line at: http://www.npl.washington.edu/av . . Preprints of papers referenced below can be obtained at: http://xxx.lanl.gov.

References

BOOMERanG:

“Mapping the CMB sky: the BOOMERanG experiment”, P. de Bernardis, et al, New Astronomy Reviews, 43, 289 (1999), and LANL preprint astro-ph/9911461, November 24, 1999.

“A flat universe from high-resolution maps of the cosmic microwave background radiation”, P. de Bernardis, et al, Nature, 404, 955, (2000).

See also the BOOMERanG Press Page on the web at: http://www.physics.ucsb.edu/~boomerang/press_images/index.html