Difference between revisions of "Dark energy"

Revision as of 17:09, April 7, 2017

The problem

In 1998, the Supernova Cosmology Project observed 42 Type Ia supernovae, most of these from the ground, in an effort to measure the rate of deceleration of the expansion of the universe.^[3] (Type Ia supernovae are objects of easily discernible brightness and thus are favorite objects for standardization of redshift and hence of the speed of expansion.) These supernovae were actually much dimmer than expected, a finding that indicated an acceleration of expansion, not the deceleration that gravitational attraction would produce. A competing group, the High-Z Supernova Search Team, reported similar results from their observations of 14 other supernovae.^[4] (The symbol z stands for redshift in this context.) The findings of an accelerated universe came as a profound surprise to all interested observers and commentators.^[5] More recent surveys have shown that the discrepancy persists.^[6][7]

Young Earth Creation Science solution

John Hartnett, in Starlight, Time and the New Physics,^[8] reminds his readers that Moshe Carmeli first formed a new model, called Cosmological Relativity, and through this model predicted that the universe would in fact appear to be accelerating. He made this prediction in 1996, two years before the publication of the Type Ia supernova data and the introduction of the idea of "dark energy" into cosmological discussions. In making this prediction, Carmeli did not invoke either dark energy or dark matter.

Hartnett extended Carmeli's model and discarded several assumptions that Carmeli initially had thought were safe. This included the assumption that the matter density of the universe is at the critical level for a "coasting" universe.

The full derivation of Hartnett's field equation that describes the motion of far-off objects is included in Appendix 2 of his book. Briefly, Hartnett begins with Moshe Carmeli's Cosmological Relativity, which adds a dimension of the radial velocity of a far-off object to the Einsteinian dimensions of space and time. This radial velocity is related to the distance of the object by this equation:

where is a constant (evaluated at 4.28 * 10¹⁷ s) that is the reciprocal of the Hubble factor H₀ in weak gravity.^[9]

Adding this dimension requires adding a new term to the classic space-time interval, and so:

where r is the distance of the object from earth (or more properly, our galaxy).

An observation of a far-off object is typically made at a given moment and from a given place, and so ds=dt=0. So the above equation simplifies to:

Carmeli's solution is:

where is the mass-energy density fraction of the universe. Specifically, , where is the critical mass density, above which the universe would be closed and destined to collapse.

The integral of the above equation, expressed in dimensionless numbers, is:

where = is the radial velocity of the object, as a fraction of the speed of light. This fraction is itself a function of the redshift of the object (symbol: z):

The key fact is that the matter density anywhere in the universe is also a function of redshift. In the simple case of "flat" space,

where is the weighted-average mass-energy density of the universe in the present epoch.

Hartnett tested these equations against the High-Z Supernova Search Team data. To do this, he used these equations to convert to an absolute magnitude:

Here,

is an independent luminosity distance, and

where is the highest absolute magnitude of any given supernova and is expressed in megaparsecs.

Recall that is a function of z, and r is sensitive to the mass-energy density fraction.

Hartnett plotted the difference against redshift for the supernovas studied most recently by Riess and Astier.^[6][7] He then adjusted the parameters and to achieve the best statistical fit to the observations. He obtained these values:

and

The second value is consistent with a mass-energy density that is four percent of critical. This is about twice as much matter as Fukugita et al. have determined that the universe has within it, but also at the upper limit of the range of that value.^[10] It is also the same as the fraction of mass-energy in the universe that most evolutionists now believe to be composed of ordinary or baryonic matter (see below).

Hartnett's model thus predicts as much mass-energy as other astronomers have already estimated that the universe has, and no more, within the limits of that estimate.

Evolutionist/uniformitarian view

The dark energy concept

Reiss, Astier, and their respective teams did not use this model. Instead, they relied upon the Friedmann-Lemaître cosmological model. That model predicted far more mass-energy than the universe possesses, beyond any rational estimate.

Saul Perlmutter, Michael Turner, and Martin White appear to have coined the term dark energy to name the phenomenon that is causing the apparent acceleration that they found.^[11] The term appears again in the more comprehensive paper of Bahcall, Ostriker, Perlmutter, and Steinhardt, that proposes that a heretofore unsuspected form of energy "overcomes the gravitational self-attraction of matter and causes the expansion [of the universe] to speed up."^[12][13]

In 2001, Reiss and his colleagues used the Hubble Space Telescope to capture on film the furthest supernova then seen, SN1997ff, at a distance of 10 billion light-years. The magnitude and redshift of this object were consistent with a decelerating expansion. This is consistent with the model that Reiss and others were forming at the time, stating that the expansion of the universe was initially decelerating and later accelerated after its matter density dropped below a critical level.^[14]

In 2003, a survey of 11 Type Ia supernovae by the HST confirmed the earlier findings of an accelerated expansion of the universe. Astronomers consider this the most definitive evidence to date for the existence of dark energy.^[15]

Nature of dark energy

Naturalistic cosmologists estimate that 70 percent of the total energy in the universe consists of dark energy.^[17] Dark matter occupies another 25 percent. The remaining portion is the familiar, or baryonic matter of which all objects are made.^[18]

Uniformitarian cosmologists have three theories of what form this energy might take:

1. It is a fundamental property of the universe, as Albert Einstein originally suggested. Einstein's original idea was that this force, which he called a "cosmological constant," would exactly counterbalance gravity and thus keep all galaxies and other objects of similar size in the same place. Einstein initially discarded his own findings after Edwin Hubble showed that the universe was expanding. Some modern cosmologists suggest that Einstein might have been right after all.^{[13][14][16][17][18]}
2. It is a previously unknown type of energy fluid or field, and perhaps even a fifth elemental force, in addition to the previously known four forces of gravity, the electromagnetic force, and the weak and strong nuclear forces. Some scientists name this new force "quintessence" (literally, fifth essence), a term that ancient Greek philosophers once coined for a fifth "element" of nature in addition to the four elements that they thought they knew (fire, air, earth, and water).^[13][17]
3. It is not a new property or force but a manifestation of an error of our understanding of an old one, namely gravity.^[13][17]

Implications of dark energy

Three suggested fates of the universe

This has not stopped them from speculating on what might happen to the universe in the future. They currently recognize three possibilities:^[18]

1. The universe will expand indefinitely and isolate our galaxy.^[16]
2. The "quintessential" substance will reverse its repulsive effect and become attractive. This will stop the expansion and contract the universe into a point, an event they call the "Big Crunch."
3. The universe will expand rapidly enough to tear the fabric of space and ultimately cause all baryonic matter to disintegrate, an event they call the "Big Rip."

Other scientists raise the question of whether the proportion of dark energy in the universe is a fundamental quantity of space. If it is not, they say, then this universe is only one of many.^[20]

Criticisms of the dark energy concept

Creationist criticism

Dr. Walt Brown, a young earth creationist and opponent of atheistic evolution, wrote about the concept of dark energy:

Dark energy is a mathematical convenience. Some authorities admit that it may never be detectable. One reason why no one would ever detect it is that it represents an error. Evolutionistic cosmologists have admitted that dark energy might represent an error of their understanding of gravity. Creationists suggest that the error is far more fundamental.

Secular criticism

David Cline wrote in Scientific American in 2003 the following:

A Competing Secular Explanation

Timothy Clifton, at Oxford University, said in an interview published at Space.com on October 1, 2008 that the solar system might be enclosed in a volume of space having a disproportionate lack of matter. According to his "void model," light from far-distant objects appears dim, not because the objects are retreating from the earth or even because the cosmos is expanding, but because the void condition does not focus the light as a matter-filled cosmos normally would. Clifton expresses one misgiving about his model that clearly reveals his worldview, however: it would negate the so-called Copernican Principle and require astronomers and cosmologists to regard the earth as a special place.^[24]

Proposed investigations

Fermilab's Experimental Astrophysics Group has proposed a Dark Energy Survey, using a special 500-megapixel camera mounted on a ground-based telescope.^[25]

NASA and the United States Department of Energy have also proposed a Joint Dark Energy Mission, essentially a new, very-high-resolution space telescope. Three different telescope designs are now under study.^[26] Timothy Clifton, the proponent of the "void model" mentioned above, hopes that observations by JDEM might provide a reliable test of his model.^[24]

External links

• Relativity Calculator - Glossary

References