Re-Evaluating Our Cosmological Models: Why Now?

In my 1964 Science Fair project, entitled 'The Structure of the Universe'  (which was given a feature look in the Miami Herald) I got many things wrong. The reason wasn't to do with errors, but in using the existing base of cosmological data and information to construct my model. Chief among these was the theory of continual creation which had been proposed by Fred Hoyle and Hermann Bondi.. It proposed that a hydrogen atom was ‘created’ in the universe on the basis of the perfect cosmological principle. A quantitative rate for the input-creation advanced by Jayant Narlikar ('The Structure of the Universe', Oxford Univ. Press, 1977) was:

4.5 x 10^-45  kg m^-3 s^-1

This was taken to be the rate of new matter created per second within a cube - which is expanding at the rate H, where H is Hubble's constant. Then, one second later the side dimension of the cube will have increased by (1 + H)  and its volume will have increased to (1 + H)³ .  In this way, new matter is created within the 1 s interval with new mass: M = 3H r.

 And so,  though the universe was indeed expanding, it didn’t change its appearance. So its density must remain the same.  (The additional space created by the expansion must therefore have the same density of matter, r )   In addition, because of the principle of “continual creation”, the universe had no beginning and no end.  Thereby I was able to construct a model based on a matter and anti-matter universe (one with positive curvature the other, negative)  in a state of "equilibrium" with matter destroyed via annihilation equal to the new matter created via continuous creation.

It was a beautiful model which garnered top awards, but alas only months away from becoming passé.  This transpired when the first  evidence for the Big Bang emerged. This was thanks to Nobel-winning work by Arno Penzias and Robert Wilson. The experience showed me (as it did Fred Hoyle and Hermann Bondi) that our perspective on the universe and especially models, can change with just one major new discovery.

I based a lot of my model on the validity of the perfect cosmological principle which maintained that the universe was the same in space as well as time, and the same physical laws that apply on Earth applied everywhere else. In other words, our solar system and planet are nowhere special. Two sub-assumptions of the principle are that: 1)  the universe is homogeneous, i.e. looks the same for all locations, and 2) the universe is isotropic, appearing the same in all directions.

But back in the 1960s we still didn't know of the existence of cosmic voids. Those had to wait five decades for their discovery. Voids have roughly 1/10 the matter density of galaxy clusters (like our Local Group) but account for nearly 60 percent of the volume of the visible universe, thereby introducing inhomogeneity.

Even before the void discovery, there was the discovery of relic structures of the Big Bang by George Smoot and his collaborators at the University of California at Berkeley, in 1992. The investigation made use of data obtained from NASA's Cosmic Background Explorer (COBE) satellite. The data exposed very small temperature differentials (dT), from which density variations could be deduced. (In principle the temperature variations of the form dT/T are taken as a proxy for density fluctuations (dr / r)  in the early universe). These variations were also  found consistent with the postulated characteristics of an inflationary cosmos, as opposed to an always uniformly expanding cosmos. Indeed, an inflationary phase would feature an exponential rate of expansion by way of doublings over very small time periods.

What is the problem? It has remained trying to model a homogeneous universe despite data and findings that show the universe is inhomogeneous.  To quote astrophysicist Thomas Buchert (New Scientist, June, p. 33):

"To model such a complex structure with a homogeneous solution is a bold idealization."

Of course, cosmologists haven't been deterred. They merely resort to what's called modeling via  "statistical homogeneity" which means upping the scale for examining the cosmos to one wherein the inhomogeneities are radically reduced or vanish.  For example, on the scale of 400 million light years, voids and galaxy clusters average out into uniformity. But is this 'kosher'? Probably not because we have no real visualization of the cosmos on such scales.

Not yet mentioned are dark matter and dark energy, especially how the latter overturns our conceptions of cosmic order, see e.g.

http://brane-space.blogspot.com/2008/10/dark-energy-evidence-for-new-law-of.html

Dark energy has also been found to be linked to the accelerated expansion of the cosmos, e.g.

http://brane-space.blogspot.com/2011/10/three-share-physics-nobel-for.html

Even more interesting, the cosmos' inhomogeneity contributes separately to the acceleration. Thus as more mass has clumped into galaxy clusters, the cosmic voids have grown causing the universe to expand more rapidly in those regions. The result is an accelerating effect similar to that attributed to dark energy but without any remote hint of it. (See e.g. The Journal of Cosmology and Astroparticle Physics, Vol. 10, p. 043).

What does all this mean for our cosmic perspective and cosmological models? Headaches! It means we may have to ditch the simplistic idealizations that pander to order, uniformity and aesthetics and instead come up with some ugly alternatives that violate our temperaments. For example, the whole Einsteinian notion of space-time is predicated on a continuum in which the entities are conjoined. But....if space expands at much faster rates in certain places then one must accept that clocks will tick at different speeds too.

As incredible as that sounds, it doesn't come near the ultimate conclusion: that if this is so it means the very age of the universe (which we now give as 13.8 billion years) is not a constant and instead will depend upon where the measurement is made. If you measure within a void you will get one answer, and in a galaxy cluster another. (According to one recent theory, it implies the age of the universe would be measured to be up to 18.6b years old where the low density of matter "means the clock has ticked particularly fast", New Scientist, op. cit. )

But which is better? To live with our idealistic fantasies of order and uniformity of space-time, or to live in reality and know the actual truth of how the universe operates?   Bear in mind the entire history of our science has been overturning sundry pet concepts of the universe, and especially our place within it.

Now may be the time for cosmologists to put on their big boy pants and devise theories which, although they may try the orderly temperament, are much closer to reality!

Dark Energy - Evidence for a New Law of Physics?

Before 1998, few if any astronomers had heard of “dark energy”. Rather, “dark matter” had come to the fore with a series of articles in various periodicals, journals (e.g. Physics Today, (1992), Vol. 45, No. 2, p. 28 by S. Tremaine) Dark matter was acceptable to most of us because at least it could be understood easily at some level. After all, Fritz Zwicky in 1933 actually laid the original, observational basis for dark matter. His measurements of galaxy clusters highlighted a 'missing mass'. He found that the mass needed to bind a cluster of galaxies together gravitationally was at least ten times the (estimated) apparent mass visible.

This mass, because it was inferred but not directly detectable, became the first dark matter. Around the same time there were other confirmations, based on observed stellar motions in the galactic plane by Dutch astronomer Jan Oort. He determined there had to be at least three times the mass visibly present in order for stars not to escape the galaxy and fly off into space.

By the time of Tremaine’s Physics Today article, it was estimated that at least 90% of the universe was in the form of dark matter, and barely 10% constituted visible matter – meaning that it either reflected radiation or emitted it at some wavelength. Many of these results issued from the data acquired by the Cosmic Background Explorer (COBE) satellite.

By 2000, this whole picture had radically changed and new assays for the mass-energy distribution for the universe had ordinary visible matter at only 7% of the total, with fully 93% a “dark component” - of which nearly 70% was dark or vacuum energy, the rest dark matter. (See, e.g. Physics Today, July, 2000, p. 17)

This was almost too much to take. Of course, as a physicist (in solar physics at that time) I’d been familiar with the claim of vacuum energy in various crank forums, or via e-mails from cranks. Most of them embraced the notion that empty space was replete with vacuum energy at an almost infinite density level – and accessible if one can only get to it. Free energy without the hassle of infrastructure!

In no way did anyone – even those astronomers least conversant with modern cosmology – expect the most distant objects to exhibit a slowing down, and the closer ones to exhibit a speeding up: indicating the expansion was accelerating, and worse, that a counter (repulsive) force to gravitation might be operating. However, not when two separate groups find supernovae that are dimmer – and thus further away – than they should have been,

But in science, meticulously obtained and plotted data seldom lie. And by early 1998, the type Ia supernova results of two groups: the Supernova Cosmology Project (based at UC Berkeley) and the High- Z Supernova Search - led by Brian Schmidt of Mt. Stromlo Observatory in Australia, began to show tightening error bars.

Why Type 1a supernovae? First, because they’re bright enough to isolate in different galaxies – hence there’s a cosmological dimension. Second, they exhibit a uniform, consistent light spectrum and brightness decay profile (all supernovae diminish or ‘decay’ in brightness after the initial explosive event). This applies to all galaxies in which they appear so they function as cosmic standard “candles”. Third, all Type 1a’s betray the same absorption feature at a wavelength of 6150 Angstroms (615 nm) - so have the same spectral “fingerprint”.

(See Figure 1)



Basically, the majority of plotted Type 1a supernovae data points congregated along the upper of the two plot lines shown in Figure 1. (One sample point is shown) This placed them firmly in the region of the graph (of observed magnitude vs. red shift) we call “accelerating universe”. On the other side of the diagonal is the "decelerating region". An additional feature of the accelerating side is 'vacuum energy'.

While my first instinct was to reject the notion of vacuum energy, this didn’t withstand further examination. The bottom line is that the best fit to the supernova data indicate that the energy density of the vacuum translates into a repulsive force that can counter gravity’s attraction.

To get an insight, we can examine the equation that underpins cosmic expansion and whether it is accelerated or not (cf. Perlmutter, Physics Today, 2003)

R"/R = - {4pi/ 3} G rho (1 + 3 w)

Here R is a cosmic scale factor, R" is the acceleration (e.g. second derivative of R with respect to time t), G is the Newtonian gravitational constant, rho the mass density. We inquire what value w must have for there to be no acceleration or deceleration. Basic algebra shows that when w = -1/3 the whole right side becomes zero. The supernovae plot data constrains w such that it cannot have a value > (-1/2). Most plausibly, w, the ratio of pressure to density is (Perlmutter, ibid.)

w = (p / rho) = -1

This is consistent with Einstein's general theory of relativity - which one could say approaches the status of a 'basic law of physics'. In this case, a negative pressure meshes with general relativity's allowance for a "repulsive gravity" - since any negative pressure has associated with it gravity that repels rather than attracts.

Some might argue that cosmic repulsion shows a "new law" of physics, but it's merely extending the existing concept of gravitation to show it has a repulsive as well as attractive aspect, and has always been consistent with Einstein's general theory of relativity.

This brings us to the question: What’s to become of the cosmos if the acceleration is ongoing? Clearly, photons emerging from whatever cosmic object (star, nebula etc.) can never catch up to the (too) rapidly expanding space-time. This means that over time, fewer and fewer objects will be visible to any sentient observers. Eventually, all cosmic objects will “vanish” from the scene and all observers – if any remain- will be plunged into featureless skies.

(To be continued)