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There’s a Hole in Bottom of the Universe!

by John G. Cramer

Alternate View Column AV-141
Keywords: cosmic, microwave, background, CMB, cold, spot, WMAP, supervoid, negative, mass
Published in the April-2008 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted
10/02/2007 and is copyrighted ©2007 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

 

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It’s perhaps natural to think that our universe should be more or less the same in all directions, once we average out the lumpiness of stars, galaxies, galactic clusters, superclusters, etc.  However, there’s a growing body of evidence suggesting that this presumption is not true.  There is now a strong suspicion that our universe may contain a gaping “hole” located in the constellation Eridanus.  This all started several years ago with the observation that there was a pronounced “Cold Spot” in the data from the Wilkinson Microwave Anisotropy Probe (WMAP) that produced space-based measurements of the cosmic microwave background (CMB) left behind about 400,000 years after the Big Bang.


Let’s start by reviewing the cosmic microwave background.  Shortly after the initial Big Bang, when fast exponential inflation had stopped, our universe settled down to a slower and steadier rate of expansion.  As more space became available to hold the energy in it, the universe cooled to a nearly perfect “liquid plasma” saturated with energy, in which quarks and gluons behaved as free particles.  As the cooling progressed, the gluons thinned out and the quarks clumped into composite mesons, protons, and neutrons.  For some reason that remains obscure, there was a slight excess of protons and electrons over their antimatter equivalents (antiprotons and positrons).  During the high-density stages of the early universe, nearly all of the antimatter particles paired off with their matter counterparts to annihilate, leaving behind the surviving matter particles and producing a universe populated almost exclusively by matter.  The cooling universe was then a “soup” dominated by free electrons and protons.  In this environment, a photon of light, strongly influenced by any encounter with a charged electron or proton, could travel only a short distance without being absorbed or scattered by one of the free charged particles.  But as the cooling progressed, the negative electrons and positive protons tended to pair off to make electrically neutral hydrogen atoms. The free charged particles, which easily absorb photons, were replaced by light-transparent neutral atoms.  The murky black “soup” of the early universe became crystal clear.

The photons of the early universe had energies characteristic of the light emitted from a hot object (the universe) at a temperature of about 2,900 K.  (Here, K means “kelvin” and specifies the absolute temperature in Celsius degrees above absolute zero.)  As long as the universe was murky black, these photons were trapped by repeated emission and re-absorption.  However, the transformation to a transparent universe released them from this trap, and they became free photons.  These liberated photons have been traveling through the universe ever since, and we detect them today as the cosmic microwave background radiation. However, as the universe expands and space itself stretches, the wavelengths of these CMB photons were also stretched until they are now microwave photons characteristic of a very cold object with a temperature of 2.73 K instead of visible light photons characteristic of a hot object with a temperature of 2,900 K.  We observe these CMB photons today as microwaves emitted from a “surface” that has not existed for the last 13 billion years.

There has been a recent flurry of activity to sweep the sky and map the CMB photon intensity vs. angular position on a fine scale.  WMAP has produced such a mapping, producing an orange-tinged bluish map has become well known in science articles and book cover art (see the figure below).  In a localized area on the right and well below the map’s center there is a particularly cold region, quite dark as compared to the blue, yellow, and orange regions of most of the rest of the map.  This region is called the “WMAP Cold Spot”, and it lies in the river constellation Eridanus.

When a feature like this is obvious to the naked eye, there is a fairly good chance that it is significant.  The question that is raised is whether the Cold Spot is just an expected fluctuation in the intensity of the CMB, or whether it is non-statistical and might be an indication of something particularly interesting going on.  The answer is that it is definitely not a simple statistical fluctuation.  A group working the Physical Institute of Cantabria in Spain and at Purdue University has carefully analyzed the WMPA data and has concluded that the Cold Spot is not compatible with normal Gaussian fluctuations of the CMB.

Recently two groups, one at the University of Minnesota and the other at Cavendish Labs in England and in Lausanne, Switzerland, have carefully examined the data from the National Radio Astronomy Very Large Array Sky Survey (NVSS), which looked at 82% of the sky visible from the VLA in New Mexico and catalogued more than 1.8 million individual radio sources.  The groups studied this extragalactic survey, looking for structure at the Cold Spot location.  Both groups have found a sizable dip in source population and radio brightness at just the location of the WMAP Cold Spot.

The implication of these results is that the Cold Spot is not a characteristic of the CMB itself, but instead it is a phenomenon that happens as the CMB photons pass through the universe on the way to our detectors. The combined WMAP and NVSS data suggest that along the line of sight to the Cold Spot, there is an enormous volume containing almost no stars, galaxies, or gas.  A physical process called the integrated Sachs-Wolfe Effect, the gravitational wavelength shift of photons as they pass through varying gravitational fields in an expanding universe, is probably responsible for the cold spot.  As photons of light fall into the gravity well of a massive object like a galactic cluster, they gain energy and are blue-shifted.  On emerging from the gravity well, such photons would lose the energy gained, except that, due to the accelerated expansion effect of the large quantity of dark energy in the universe, there is a net repulsion acting and it is a bit easier to get out of the gravity well, so that not all of the gained energy is removed.  The net result is that CMP photons that pass through regions containing significant mass arrive at our detectors with a bit more energy on the average than those passing through regions of the universe that are relatively empty.  Therefore, the CMB radiation should appear cooler along a line of sight passing through a large “empty” region.  Ineffect, the CMB radiation is weighing the universe along the various lines of sight.

Although sizable empty regions of the universe have been observed before in deep-sky surveys, the region that has produced the Cold Spot appears to be much larger.  It appears to be an unusually large “supervoid”, perhaps 1000 times larger than the largest empty regions previously detected. The Cold Spot Supervoid is estimated to be around 6 to10 billion light years from the Earth, at a red-shift factor of about z=1,  and to have a diameter of around one billion light years.  It is perhaps worth noting that no computer simulations of the formation and evolution of the universe have ever predicted a void of such a size.


What could cause the Cold Spot Supervoid?  I have not seen any speculations in the astrophysics literature as to its origin.  The prevailing view seems to be that if it is there, then “it just happened.”

However, since this is a science-fiction magazine, let me indulge in a bit of SF-related speculation.  Some years ago I recall having a discussion about negative mass objects and cosmic voids with my good friend, the late Dr. Robert W. Forward.  Bob Forward, for reasons that those familiar with his work will understand, was interested in the possibility that large concentrations of negative mass might exist in the universe.  He noted that if there happened to be an object somewhere in the universe, perhaps a natural worm-hole mouth, that had a very large negative mass, then it would tend to repel all of the positive mass in the region, pushing it far away and sweeping out a large empty region in the universe.

Since we now know about the dominant dark energy in the universe, we can now add to Bob Forward’s speculation by noting that the integrated Sachs-Wolfe Effect would work backwards for photons that were climbing the gravity “mountain” of a negative mass object (the inverse of a gravity well) and would cool the photons passing through a region dominated by negative mass.

We have learned from general relativity that, given some quantity of negative mass, we could build space-time metrics that allow one to do all sorts of cool SF-related faster than light gymnastics.  Therefore, if we need some negative mass to construct wormholes, warp drives, Krasnikov tubes, and so on (see earlier AV columns in this series), there is now a good place to look for it.  Just get in your hypervelocity starship and head for the WMAP Cold Spot.

 


AV Columns Online: Electronic reprints of about 145 "The Alternate View" columns by John G. Cramer, previously published in Analog, are available online at: http://www.npl.washington.edu/av.

 


References

The WMAP Cold Spot:

“Detection of non-Gaussian Spot in WMAP”, M. Cruz, E. Martinez-Gonzalez, P. Vielva, and L. Cayon, Mon.Not.Roy.Astron.Soc. 356  29-40 (2005), available online at http://www.arxiv.org/PS_cache/astro-ph/pdf/0405/0405341v2.pdf

 “ExtragalacticRadio Sources and the WMAP Cold Spot”, L. Rudnick, S. Brown, and L. R. Williams; Astrophysics Journal (2007, to be published); available online at http://www.arxiv.org/pdf/0704.0908

 “Probing dark energy with steerable wavelets through correlation of WMAP and NVSS local morphological measures”, J. D. McEwen, Y. Wiaux, M. P. Hobson, P. Vandergheynst, and A. N. Lasenby, 2007, submitted to Mon.Not.Roy.Astron.Soc.,   available online at http://www.arxiv.org/PS_cache/arxiv/pdf/0704/0704.0626v1.pdf .


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