This is a summary of my (imperfect) understanding of the essay of the same title by Jeremiah P. Ostriker and Paul J. Steinhardt in Scientific American, January 2001, pp. 4755.
Dark Energy, which accounts for 70% of the total mass in the Universe, and which causes the recently observed acceleration in the expansion of the Universe, comes in two varieties (both assumptions):
- Vacuum energy (Einstein’s cosmological constant), which is static and constant, just so much per unit of space, but of course a growing total as space expands.
- Quintessence, which varies slowly, up or down, and is a dynamic quantum field. A further elaboration specifies the “tracker field”, which for a stretch of cosmic history tracks the decreasing energy density in the Universe.
Both types originate from the quick generation and annihilation of virtual particles in “empty” space.
There are 3 numbers (not either 6 or 9 as in two recent books) that characterize the Universe, illustrated in a “cosmic triangle”: relative density of dark energy, relative density of dark matter, and the curvature of space-time. It appears that the curvature is = 1, i.e. space is flat, not either hyperbolic or spherical.
Recipe for the Universe.
| Dark energy | 70% |
| Exotic dark matter | 26% |
| Ordinary dark matter | 3.5% |
| Visible matter | 0.5% |
| Radiation | 0.005% |
| ——- | |
| 100.005% |
Exotic dark matter, according to other authors, is called “hot dark matter” (mainly neutrinos) or WIMPS (weakly interacting (nearly) massless particles).
Ordinary dark matter is also called “cold dark matter”, mainly brown dwarfs or Jupiter-sized planets, or MACHOS (massive astra-like cold halo objects, found in the halos of galaxies. ‘
Visible matter consists of stars, galaxies, quasars, planets, and luminous interstellar clouds. “Visible” means not only by visible light, but also by ultraviolet, X-rays, infrared, or microwave; in fact the whole electromagnetic spectrum.
As I said before, quantum fields in “empty” space generate pairs of virtual particles. These fluctuations contribute positive (repulsive) energy (explosion) if the virtual particles are fermions (since fermions obey the Pauli exclusion principle and so cannot occupy the same (quantum) space at the same time, by Fermi-Dirac statistics), but contribute negative (attractive) energy (implosion) (ordinary gravity) if the virtual particles are bosons. Fermions (e.g. electrons and quarks) have halfintegral spins, while bosons (e.g. photons and gluons) have whole-integer spins. Fermions are particles of matter while bosons carry the forces between fermions.
There is not an exact cancellation of the positive and negative energy, i.e. antigravity (dark energy) and gravity. However, super symmetry (a theory that postulates that each particle has a superpartner of the different type of spin) would very nearly balance them, to 120 decimal places; but supersymmetry comes into play only at very high temperatures, which may have been present at the Big Bang origin of the Universe. This near-balancing seems like another example of fine tuning, which made galaxies, stars, planets, and life possible in our Universe. This could be justified by the anthropic principle (if it were not so, we would not be here to wonder about it, and perhaps in other universes we are not there), but another scenario is possible, as explained below.
An early event due to the overbalancing of antigravity over gravity could be the stipulated early “inflation” (super-rapid expansion or explosion) of the Universe, which occurred at 10 to the minus 35 seconds after the Big Bang. When inflation ended, quintessence (now the preferred version of the dark energy) achieved balance with gravity and expansion of the Universe slowed down considerably. After that, the decrease in the dark energy density tracked the decrease in the density of (ordinary) energy, until the Universe was about 30,000 or 300,000 years old, when matter and energy decoupled and the Universe became transparent to radiation. This tracking (hence “tracker field”) actually continued until almost the present age, when the Universe was 5 billion years old. (It is now 15 billion years old, which is not much later than 5 billion on a logarithmic scale.) So since 5 billion years ago, when the density of total matter began to exceed the density of energy, the tracking by quintescence density of energy density stopped and changed sign again, to initiate the present trends of accelerated expansion of the Universe, which may continue at an exponential rate forever, unless quintessence changes its sign again.
If accelerated expansion continues forever at an exponential rate, galaxies and stars will move beyond the distance where communication (even at the speed of light) is possible, and there may be only isolated islands of life that don’ t (can’f} know about each other (so what’s new?), or: no life at all if everything gets too cold. If quintessence changes sign again, the Universe may contract back to its present size (or beyond?), and the age of galaxies, stars and life may reappear; but they will never know that a preexisting age of stars and life was ever here. Or are we already that second (or even nth) generation?
This is a pulsating model of the Universe, not necessarily through a previously postulated alternation of Big Bang and Big Crunch, and not necessarily my previous model in “Ages of the Universe” involving a “supersYmmetry transformation” from a supercold Bose-Einstein condensate to a super-hot Big Bang unstable fermion ball. Either version of the pulsating Universe would not predict a dead end of life and humanity; but the version proposed here would do so only if quintessence is the correct model of the dark energy. If dark energy is a static invariable “vacuum energy”, the pulsation could not occur, and the dead end would be final.
A pulsating Universe was long ago postulated, without benefit of modern cosmology or quantum physics, by ancient Vedic philosophy.