Cosmic Fog
November 16, 2012
There were two very significant
cosmological discoveries in the
last century. The first was the observation of
universal expansion by
Edwin Hubble and
Vesto Slipher. The second was the discovery, by
Arno Penzias and
Robert Wilson, of the
cosmic microwave background radiation. This was direct evidence for the
Big Bang. This microwave background radiation was measured in great detail by the
Cosmic Background Explorer satellite.
Arno Penzias (front) and Robert Wilson (rear), standing at the horn antenna with which they discovered the cosmic microwave background radiation. Fortunately, ubiquitous cellphone cameras document today's Nobel-worthy discoveries in higher resolution. (Via Wikimedia Commons)
The cosmic microwave radiation is a cosmic fog at
microwave frequencies, since it pervades the entire
universe and exists at a detectable background level.
Astronomers have now examined another pervasive cosmic fog, more
scientifically called
extragalactic background light (EBL), using
gamma rays.[1-4]. This cosmic fog consists of
photons from
early stars formed about 400 million years after the big bang, plus all light created thereafter. This observation was made by the
Fermi Gamma-ray Space Telescope.
To paraphrase a
famous quotation, "Old photons never die, they bounce around the universe forever," thus forming the EBL. Or, as expressed in a
NASA press release, the EBL contains "the total amount of light from all of the stars that have ever shone."[2] Like any background, the EBL sets a threshold of detectability for objects within it. As a result of the Fermi Telescope observations, this allowed an estimate of the average
density of stars in the universe; and, thereby, the average distance between stars. The density is 1.4 stars per 100 billion
cubic light-years, and the average stellar spacing is 4,150
light-years.[1-4]
The Fermi Telescope team was able to make these estimates through study of 150 gamma ray
blazars. These are intense sources of gamma rays, with energy greater that three
GeV.[2] These gamma rays are created as matter falls into massive
black holes within
galaxies. The gamma ray photons have a probability of interaction with photons of the EBL and conversion into an
electron-positron pair.[2]
The green dots show the locations of the 150 blazars in the EBL study. The plane of our Milky Way Galaxy runs through the middle.
(Image: NASA/DOE/Fermi LAT Collaboration).[2)]
The research team measured blazars out to a
redshift of about 1.6, corresponding to a time when the universe was just four billion years old. [1] The youngest blazers that were measured existed at about eleven billion years after the Big Bang.[3] As shown in the graph, they were able to determine the average gamma-ray
attenuation for three distance ranges between 9.6 billion years ago and the present time, thereby allowing an estimate of the EBL and the average stellar density.[2]
The more background light, the greater the attenuation. As the universe evolved, gamma ray absorption became more extreme. (Image: NASA's Goddard Space Flight Center).[2)]
References:
- M. Ajello, R. Buehler, A. Reimer, et al., "The Imprint of the Extragalactic Background Light in the Gamma-Ray Spectra of Blazars," Science, Advanced Online Publication, November 1, 2012, DOI: 10.1126/science.1227160.
- Francis Reddy, "NASA'S Fermi Measures Cosmic 'Fog' Produced by Ancient Starlight," NASA Goddard Space Flight Center Press Release, November 1, 2012.
- Thomas H. Maugh II, "Astronomers measure cosmic 'fog,' estimate space between stars," Los Angeles Times, November 1, 2012.
- Megan Garber, "Astronomers Read the Shadows of the Universe's Earliest Stars," The Atlantic, November 1, 2012.
- Cosmic 'Fog' Produced by Ancient Starlight Measured By Fermi, YouTube Video by NASA's Goddard Space Flight Center/Cruz deWilde, Nov 1, 2012.
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