|
|
This Chandra
X-ray Observatory image shows the galaxy cluster Abell 2029.
Its distance corresponds to a light travel time of 1 billion years. The
red diffuse emission (visible only in X-rays) shows hot intergalactic
gas, heated to about 100 million degrees by the enormous gravity in the
cluster. The distance can be derived by
calculating the relative amounts of hot gas and dark matter and shows
that
the expansion of the Universe began accelerating about six billion
years
ago. |
The uncomfortable truth? |
| The recent discovery that the expansion of our observable universe, instead of coming to a modest slow-down is accelerating further, has stirred some unrest among cosmologists. For decades, the preferred guess has been a final halt of expansion at the end of time. With the new observation that very distant supernovae seem to be dimmer than expected, such a peaceful outlook has become rather unlikely. Not only that we feel uncomfortable with that apocalyptic perspective, it also represents a challenge for theoretical physicists. In former decades, we mainly discussed to which extent the expansion of the universe was decelerating: Would it revert to contraction? Would expansion come to a halt? Or would it never come to a halt? The fact that the universe was actually accelerating its expansion came as a surprise and required a major revision of our cosmological view of the world. |
| One of the few physical laws most of us will remember from secondary school is, that a body changes its velocity if it is under the influence of a force. The intuitive expectation of a decelerating expansion rate of our universe follows from our life-long experience with gravity, the force that was first described mathematically by Sir Isaac Newton and that drives apples to fall from trees and keeps our earth in orbit around the sun. The conclusion was not too far-fetched that it will finally somehow keep together also our expanding universe. However, since we now diagnose an increase (and not a decrease) in the recession velocity of distant galaxies, some other forces less amenable to intuition seem to be operative. |
| It did not take too long and a new tasty term was found to designate the source of this repulsive force: "dark energy" (in essence a codeword for something unknown, Dvali 2004). And although these new developments had not been greeted with much enthusiasm in the beginning, in the meantime a majority of cosmologists seems to believe that this newly postulated Dark Energy thing makes up about 70% of our actual universe. Rarely before, a new cosmological concept has been widely accepted in such a short time. |
| An alternative model suggests that inflation, the mechanism made responsible for blowing up our universe from particle physics dimensions to macroscopic dimensions 10-30 sec after the Big Bang [see my earlier essay] has proceeded on a very modest scale up to present times. In that case, our impression of increased recession of distant galaxies would be due to the continued increase in the extension of spacetime and not to a change in the velocities of these galaxies themselves. |
| Having tasted the sweetness of elementary particle physics, cosmologists lost their shyness and are ready now for rather immodest proposals. E.g., they soon recognized that even in a totally empty space, quantum fluctuation should permanently lead to the spontaneous appearence and disappearence of particle / anti-particle pairs. A tentative estimate of this "vacuum energy" resulted in a number roughly 120 orders of magnitude above the energy density of ordinary matter. However, they stand back from equaling this quantity to the mysterious force accelerating the expansion of the universe, since in that case, not only would distant galaxies be accelerated, but also our earth and everything on it would be torn apart. Only the most fanatic theorists will be disappointed that, obviously, this is not the case. |
| Neither Dark Energy nor continued Inflation can count on our spontaneous comprehension. In both scenarios, assumptions far from our every-day experience have to be made. This pill is difficult to swallow for an entity that is supposed to be responsible for 70% of our reality... Hopefully other, more orthodox explanations will be found (Ellis 2008). |
| 6/05 < MB
6/05, 4/08 > 7/05 |
| More... |
| G.Dvali (2004) Neutrino probes of dark energy. Nature 432: 567-56 |
| G.Ellis (2008) Patchy solutions. Nature 452: 158-160 |
| J.P.Ostriker, P.J.Steinhardt (2001) Brave New Cosmos: The Quintessential Universe. Sci. Am. Jan. 2001 |
| p.s.:
A new
idea has the potential to put cosmology back on its feet. While most
cosmologists wonder why our universe consists exclusively of matter,
Gabriel Chardin (CNRS, France) proposes that anti-matter resides in the
voids surrounded by the bubble-like filamentous sheets of ordinary
matter. Mysterious 'dark matter' and 'dark energy' might only reflect
the still enigmatic properties of this exotic sibling of ordinary
matter
overlooked for all too long. One prediction of this hypothesis was
rejected in 2023: Antimatter is attracted by ordinary matter and not
repelled (Anderson 2023). |
| G. Chardin (2020) Antimaterie im neuen Licht. Spektrum der Wissenschaft 1.20, 64-71. G.Chardin (2019) L'antimatière tombe-t-elle vers le haut? Pour la Science 498 |
| E.K. Anderson et al (2023) Observation of the effect of gravity on the motion of antimatter. Nat. 621: 716-22 |
