Reading 41

The following passage is adapted from an article entitled “Does Dark Energy Really Exist?” (© 2009 Timothy Clifton and Pedro G. Ferreira).

  1. In science, the grandest
  2. revolutions are often
  3. triggered by the smallest
  4. discrepancies. In the 16th
  5. century, based on what
  6. struck many of his
  7. contemporaries as the
  8. esoteric minutiae of
  9. celestial motions,
  10. Copernicus suggested that
  11. Earth was not, in fact, at
  12. the center of the universe.
  13. In our own era, another
  14. revolution began to unfold
  15. with the discovery of the
  16. accelerating universe. A
  17. tiny deviation in the
  18. brightness of exploding stars
  19. led astronomers to conclude
  20. that they had no idea what
  21. 70 percent of the cosmos
  22. consists of. All they could
  23. tell was that space is filled
  24. with a substance that pushes
  25. along the expansion of the
  26. universe. This substance
  27. became known as dark energy.
  28. It is now over a decade later,
  29. and the existence of dark
  30. energy is still so puzzling that
  31. some cosmologists are
  32. revisiting the fundamental
  33. postulates that led them to
  34. deduce its existence in the
  35. first place. One of these is
  36. the product of that earlier
  37. revolution: the Copernican
  38. principle, that Earth is not
  39. in a central or otherwise
  40. special position in the
  41. universe. If we discard this
  42. basic principle, a surprisingly
  43. different picture of what
  44. could account for the
  45. observations emerges.
  46. Most of us are very familiar
  47. with the idea that our planet
  48. is nothing more than a tiny
  49. speck orbiting a typical star,
  50. somewhere near the edge of
  51. an otherwise un-noteworthy
  52. galaxy. In the midst of a
  53. universe populated by
  54. billions of galaxies that
  55. stretch out to our cosmic
  56. horizon, we are led to
  57. believe that there is nothing
  58. special or unique about our
  59. location.
  60. But what is the evidence for
  61. this cosmic humility? And
  62. how would we be able to tell
  63. if we were in a special place?
  64. Astronomers typically gloss
  65. over these questions,
  66. assuming our own typicality
  67. sufficiently obvious to
  68. warrant no further discussion.
  69. To entertain the notion that
  70. we may, in fact, have a special
  71. location in the universe is, for
  72. many, unthinkable.
  73. Nevertheless, that is exactly
  74. what some small groups of
  75. physicists around the world
  76. have recently been considering.
  77. Ironically, assuming ourselves
  78. to be insignificant has granted
  79. cosmologists great explanatory
  80. power. It has allowed us to
  81. extrapolate from what we see
  82. in our own cosmic
  83. neighborhood to the universe
  84. at large. Huge efforts have
  85. been made in constructing
  86. state-of-the-art models of the
  87. universe based on the
  88. cosmological principle, a
  89. generalization of the
  90. Copernican principle that
  91. states that at any moment in
  92. time all points and directions
  93. in space look the same.
  94. Combined with our modern
  95. understanding of space, time
  96. and matter, the cosmological
  97. principle implies that space
  98. is expanding, that the
  99. universe is getting cooler
  100. and that it is populated by
  101. relics from its hot beginning
  102. – predictions that are all
  103. borne out by observations.
  104. Astronomers find, for
  105. example, that the light from
  106. distant galaxies is redder than
  107. that of nearby galaxies. This
  108. phenomenon, known as
  109. redshift, is neatly explained
  110. as a stretching of light waves
  111. by the expansion of space.
  112. Also, microwave detectors
  113. reveal an almost perfectly
  114. smooth curtain of radiation
  115. emanating from very early
  116. times: a relic of the primordial
  117. fireball. It is fair to say that
  118. these successes are in part a
  119. result of our own humility—
  120. the less we assume about our
  121. own significance, the more
  122. we can say about the universe.
  123. So why rock the boat? The
  124. trouble is that recent
  125. astronomical observations
  126. have been producing some
  127. very strange results. Matter
  128. in the universe should slow
  129. down the expansion of
  130. spacetime, but the supernova
  131. data suggest otherwise. If
  132. cosmologists accept the
  133. cosmological principle and
  134. assume that this acceleration
  135. happens everywhere, we are
  136. led to the conclusion that the
  137. universe must be permeated
  138. by dark energy that exerts a
  139. repulsive force. Nothing
  140. meeting the description of
  141. dark energy appears in
  142. physicists’ Standard Model
  143. of fundamental particles and
  144. forces. It is a substance that
  145. has not as yet been measured
  146. directly, has properties unlike
  147. anything we have ever seen
  148. and has an energy density
  149. some 10,120 times less than
  150. we may have naively expected.
  151. Physicists have ideas for what
  152. it might be, but they remain
  153. speculative. In short, we are
  154. very much in the dark about
  155. dark energy.