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Cosmic Voids and Great Walls

by John G. Cramer

Alternate View Column AV-47
Keywords: large scale universe structure Great Wall cosmic void

Published in the August-1991 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 1/12/91 and is copyrighted 1991 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

    Our universe, as it turns out, is an even stranger place that we thought. A few years ago astronomers studying the large scale structure of the universe announced the discovery of a "Cosmic Void", a vast region of space hundreds of millions of light years across, that is almost completely empty of stars and galaxies. More recently another group discovered a "Great Wall", a sheet of galaxies and galactic clusters half a billion light years across.

    Last Thursday, a front page headline in the New York Times announced "Astronomers' New Data Jolt Vital Part of Big Bang Theory".  Newly analyzed data from the Infrared Astronomical Satellite, the Times reported, show conclusively that these vast structures are not isolated oddities. Instead, the structures are normal features of our universe which has an intrinsic lumpiness that is far larger and more uneven than can be reconciled with the best current theories about when and how our universe formed.

    In this Alternate View column I want to discuss this latest crisis in cosmology which is causing a reexamination of our ideas about the universe and its formation. The developing "standard model" of cosmology has been required to account for a number of facts about the universe: (1) it's here, (2) it's expanding, (3) we observe light elements (helium, lithium) that, we assume, were produced very early in its formation, (4) we observe very uniform microwave radiation that, we assume, was produced fairly early in its formation, (5) the gravitational pull of the mass present in the universe very nearly balances the kinetic energy of its expansion, and (6) there are galaxies and galactic clusters that represent a "lumpiness" of the universe at a scale of about 104 light years, and . The standard cosmological model, which bears the ungainly label of "The New Inflationary Big Bang Scenario with Cold Dark Matter" had been able to account, at least qualitatively, for all of these facts. We thought we were on the road to gaining a good understanding of how the universe came to be.

    However, the new observations now demand that we add yet another item to this list: (7) there are cosmic voids and walls that represent a second lumpiness of the universe at a scale of about 109 light years. The trouble with this new observation is that it cannot be gracefully accommodated by the standard model. The vast size of the new large-scale structures is troubling, because in the accepted age of the universe, about 12 to 18 billion years, there isn't time for such large objects to form. The age of the universe is also in doubt.

    But before we discuss this new set of problems for cosmologists, let me describe what the standard model has to say about how the universe formed. Our universe, according to the standard model, began its existence with a very rapid exponential expansion for the first fraction of a second, before slowing to a more modest expansion rate. This had the effect of making the present-day universe very smooth and uniform. It also tied its present expansion rate to its present mass-energy content, and striking a density balance that synthesized about the right quantities of light elements. Thus the standard model was successful in dealing items 1, 2, 3, 4, and 5 on the list above.

    That leaves Item 6, galaxy formation. That required an extra assumption beyond the exponentially expanding Big Bang. The gravitational pull of matter causes a sort of "curdling" effect in smooth matter distribution. It tends to clump. When cosmologists first tried using computers to model the formation of galaxies through this curdling effect, they encountered a big problem: the gravitational force just isn't strong enough to pull together galaxies in the 15 billion years or so since the process started. This was very puzzling, until Dark Matter was discovered.

    Astronomers using the Doppler shift to measure the speeds of bright stars orbiting clusters of galaxies discovered that these orbital speeds were much too fast. There was much more mass in the galaxies that counting stars and correcting for non-luminous gas would predict. The general conclusion, which is now supported by many unrelated observations, is that the bulk of the mass in our universe is "dark matter", some unknown stuff that clumps together early in the evolution of the universe, pulling normal matter along with it, causing galaxies and stars to form, and otherwise remaining out of sight. Only about 0.5% of the mass of an average galaxy is accounted for by visible stars. Most of the remaining 99.5% is mysterious dark matter.

    With the addition of the extra gravitational force provided by dark matter, the computer simulations of galaxy formation began to work. The galaxies formed on the computer screens formed at about the right time and were about the right size. But the simulations worked only if the dark matter was "cold", in that it was deposited in the universe with no initial speed or temperature. This meant that one of the most likely dark matter candidates, neutrinos with non-zero rest mass, were excluded because they were produced by thermal process in the heat of the early Big Bang.

    And this, until a few years ago, was where things stood. The standard model accounted for all 7 items on the list, at one level or another, and cosmologists were beginning to feel that they were on the way to a deep and fundamental understanding of how our universe came to be.

    The new evidence that the universe is full of super-structures and companion super-voids has destroyed this optimism and complacency. The curdling effect of cold dark matter, because it is relatively localized, cannot produce such structures as The Great Wall. The formation of these large structures requires an additional mechanism. And now cosmologists are in disagreement about how this might be done.

    One approach is to assume that the Big Bang was not so homogeneous and the standard model would suggest, and that the large structures are a reflection of some initial chaotic inhomogeneities in the early universe. This approach, if it can be made to work, must be able to deal with the amazing uniformity of the cosmic microwave background. New data from the COBE satellite has shown no deviation from smooth spatial uniformity in this radiation, which was produced only 100,000 years after the Big Bang itself.

    Another possibility is to introduce "hot dark matter" to the recipe. There is some preliminary data from the USSR/US germanium solar neutrino detector suggesting that neutrinos may have a non-zero rest mass (I'll save a detailed discussion of this for a later column). Massive neutrinos, if the mass is in the right range, can produce just the kind of wall and void structures observed. However, they cannot produce galaxy formation as well. Thus, as one prominent astrophysicist put it, one needs to call on the Tooth Fairy twice, once to make cold dark matter for galaxy formation and once again to make hot dark matter for walls and voids. That is at least one call too many for many cosmologist wielders of Occam's Razor.

    An alternative approach to the problem uses the cosmological constant, a venerable concept that call up a bit of history. In the early 1920's when Albert Einstein began to apply his general theory of relativity to the universe a a whole, he discovered a problem. His beautiful equations told him that every universe he considered was dynamically unstable. They all collapsed on themselves from the gravitational pull of the mass they contained. In those days the "steady state" model of the universe was standard, and no one suspected that the universe was expanding. So Einstein, in order to stabilize his recalcitrant equations, added a "cosmological constant" that offset the pull of gravity. Effectively it gave empty space a slight negative energy density that caused large scale repulsion to offset the large scale attraction of gravity. The universe could then hang in a delicate balance of offsetting attraction and repulsion for eternity, in agreement with steady-state cosmology.

    A few years later Edwin Hubble, studying the Doppler shift of distant galaxies, discovered the expansion of the universe. Einstein immediately realized that he should have believed his equations. He should not have added the "bugger factor" of the cosmological constant, which is not needed to describe a universe in the process of dynamic expansion. He later called the cosmological constant his greatest mistake.

    These days, however, the cosmological constant has had a modest revival. Theorists like Steven Hawking and Sidney Coleman are approaching the problem of quantizing gravity by considering a space filled with four-dimensional wormholes. They have realized that it is not easy to make the cosmological constant, which represents the intrinsic energy density of empty space, equal to zero. Slight twitches of parameters can make the energy density negative (as Einstein did) or positive (so that it adds to the pull of gravity).

    With the discovery of cosmic walls and voids, some cosmologists have seized on the latter form. Giving space a slight positive energy density can, if it is done right, add a large scale tendency to curdle which could explain the observed structures. Cosmologists pursuing this approach are seeking some way of setting this cosmological constant as a natural consequence of the overall process rather that having to put it into the model "by hand". There is some promise that the initial inflation process could do this.

    If space itself should possess a non-zero energy, as would be implied by this approach, does that have science-fictional implications? Perhaps. In a recent AV column ("FTL Photons", ANALOG, Mid-December, 1990 ) I discussed the Casimir effect, an experimental demonstration that the energy content of space is reduced in the gap between two electrically neutral conducting plates. To those who are not comfortable with negative energy density, the Casimir effect becomes a bit easier to swallow if the "normal" energy density of space greater than zero. And could one extract the excess energy from space itself, perhaps leaving a trail of negative energy space bubbles? SF writers like Asimov, Clarke, and Forward have already suggested such possibilities.

    In any case, cosmology has just become much more interesting. Its practitioners have a large and nearly indigestible lump of new data to chew on. It's going to be fascinating to see how our view of the universe changes when this new information is included in an improved cosmological model.


Reference:

Large Scale Structure of the Universe:
John Noble Wilford, New York Times, page A-1 (January 3, 1991).


SF Novels by John Cramer:  my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .

AV Columns Online: Electronic reprints of about 177 "The Alternate View" columns by John G. Cramer, previously published in Analog , are available online at: http://www.npl.washington.edu/av.


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This page was created by John G. Cramer on 7/12/96.