The following passage is adapted from an article entitled “Does Dark Energy Really Exist?” (© 2009 Timothy Clifton and Pedro G. Ferreira).
- In science, the grandest
- revolutions are often
- triggered by the smallest
- discrepancies. In the 16th
- century, based on what
- struck many of his
- contemporaries as the
- esoteric minutiae of
- celestial motions,
- Copernicus suggested that
- Earth was not, in fact, at
- the center of the universe.
- In our own era, another
- revolution began to unfold
- with the discovery of the
- accelerating universe. A
- tiny deviation in the
- brightness of exploding stars
- led astronomers to conclude
- that they had no idea what
- 70 percent of the cosmos
- consists of. All they could
- tell was that space is filled
- with a substance that pushes
- along the expansion of the
- universe. This substance
- became known as dark energy.
- It is now over a decade later,
- and the existence of dark
- energy is still so puzzling that
- some cosmologists are
- revisiting the fundamental
- postulates that led them to
- deduce its existence in the
- first place. One of these is
- the product of that earlier
- revolution: the Copernican
- principle, that Earth is not
- in a central or otherwise
- special position in the
- universe. If we discard this
- basic principle, a surprisingly
- different picture of what
- could account for the
- observations emerges.
- Most of us are very familiar
- with the idea that our planet
- is nothing more than a tiny
- speck orbiting a typical star,
- somewhere near the edge of
- an otherwise un-noteworthy
- galaxy. In the midst of a
- universe populated by
- billions of galaxies that
- stretch out to our cosmic
- horizon, we are led to
- believe that there is nothing
- special or unique about our
- location.
- But what is the evidence for
- this cosmic humility? And
- how would we be able to tell
- if we were in a special place?
- Astronomers typically gloss
- over these questions,
- assuming our own typicality
- sufficiently obvious to
- warrant no further discussion.
- To entertain the notion that
- we may, in fact, have a special
- location in the universe is, for
- many, unthinkable.
- Nevertheless, that is exactly
- what some small groups of
- physicists around the world
- have recently been considering.
- Ironically, assuming ourselves
- to be insignificant has granted
- cosmologists great explanatory
- power. It has allowed us to
- extrapolate from what we see
- in our own cosmic
- neighborhood to the universe
- at large. Huge efforts have
- been made in constructing
- state-of-the-art models of the
- universe based on the
- cosmological principle, a
- generalization of the
- Copernican principle that
- states that at any moment in
- time all points and directions
- in space look the same.
- Combined with our modern
- understanding of space, time
- and matter, the cosmological
- principle implies that space
- is expanding, that the
- universe is getting cooler
- and that it is populated by
- relics from its hot beginning
- – predictions that are all
- borne out by observations.
- Astronomers find, for
- example, that the light from
- distant galaxies is redder than
- that of nearby galaxies. This
- phenomenon, known as
- redshift, is neatly explained
- as a stretching of light waves
- by the expansion of space.
- Also, microwave detectors
- reveal an almost perfectly
- smooth curtain of radiation
- emanating from very early
- times: a relic of the primordial
- fireball. It is fair to say that
- these successes are in part a
- result of our own humility—
- the less we assume about our
- own significance, the more
- we can say about the universe.
- So why rock the boat? The
- trouble is that recent
- astronomical observations
- have been producing some
- very strange results. Matter
- in the universe should slow
- down the expansion of
- spacetime, but the supernova
- data suggest otherwise. If
- cosmologists accept the
- cosmological principle and
- assume that this acceleration
- happens everywhere, we are
- led to the conclusion that the
- universe must be permeated
- by dark energy that exerts a
- repulsive force. Nothing
- meeting the description of
- dark energy appears in
- physicists’ Standard Model
- of fundamental particles and
- forces. It is a substance that
- has not as yet been measured
- directly, has properties unlike
- anything we have ever seen
- and has an energy density
- some 10,120 times less than
- we may have naively expected.
- Physicists have ideas for what
- it might be, but they remain
- speculative. In short, we are
- very much in the dark about
- dark energy.