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.