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Isotropy of the Universe

June 29, 2020

Today's light pollution prevents most of us from confirming an important observation by our distant ancestors; namely, the fact that the night sky is not uniform. The Milky Way appears as a long white streak in the field of stars, and it was revealed only through Galileo's first telescopic observations that the Milky Way was composed of individual stars more densely packed in their place among the fixed stars.

Arab astronomer, Al-Farghani (c.800-870), believed that the distance to the fixed stars was 20,110 Earth radii (65,357,700 miles). We now know that the star nearest to us, Proxima Centauri, is nearly 400,000 times more distant (4.244 light-years, 2.48313 x 1013 miles). Roger Bacon's description of the Zodiac from his Opus Majus

Roger Bacon's (c.1220-c.1292) description of the Zodiac from his Opus Majus. Bacon cited Al-Farghani's determination of the distance to the fixed stars in this work, which mixes astrology with astronomy.

My translation reads, "Among these principal stars are the twelve signs, by means of which certain other things are altered. These signs are, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces; they are so named because their stars are arranged in the sky as the named objects."

(PDF file of Roger Bacon's Opus Majus, via the Research Services website at Boston College.[1])

The Milky Way is now the name of the galaxy in which our Sun resides. The word, galaxy, derives from the Greek word for milky, γαλαξιας (galaxias). Just a century ago astronomers thought that our Milky Way galaxy, which contains between 100 and 400 billion stars located in a 150-200 thousand light year region of space, was the full extent of the universe. Space beyond the Milky Way was thought to be empty, but astronomical observations found some unexplained objects called spiral nebulae

Artist's conception of the Milky Way galaxy, based on data from the Spitzer Space Telescope

An Artist's conception of the Milky Way galaxy. This image is based on data from the Spitzer Space Telescope.

We live inside the Milky Way, so we can't take its photograph. However, we've determined its structure through observations of the radio emission of its contained hydrogen gas.

(NASA/JPL-Caltech image PIA10748.)

Slowly, consensus was reached that these spiral nebulae were also galaxies. This happened after the so-called "Great Debate" on April 26, 1920, between Harlow Shapley (1885-1972), director of the Harvard College Observatory, and Heber Curtis (1872-1942), a past president of the Astronomical Society of the Pacific and director of the Allegheny Observatory. Shapley argued that these objects were small bodies that existed at the periphery of our galaxy. Curtis' view, that they were large and distant independent galaxies, was more convincing.

In a fast-forward to the present day, we now believe that we inhabit a universe of at least 2 trillion galaxies that had a precise origin 13.799 ± 0.021 billion years ago in an event called the Big Bang. Such a precise age estimate was enabled by the 1964 discovery of the cosmic microwave background radiation (CMBR) by Arno Penzias and Robert Wilson. This microwave background radiation was measured in great detail in 1989-1993 by the Cosmic Background Explorer satellite observatory, which measured a thermal black body spectrum to be at a temperature of about 2.725 K.

If the universe is isotropic, the observable universe has a spherical volume centered on the observer (An extraterrestrial, if one exists, would see a slightly different universe than the one that we observe). Is the universe really isotropic? The amazingly precise measurement of the cosmic microwave background radiation by the Wilkinson Microwave Anisotropy Probe showed nearly perfect isotropy (see figure).

Map of the cosmic microwave background radiation by the Wilkinson Microwave Anisotropy Probe.

Fluctuations of the cosmic microwave background radiation as revealed in seven years of measurements by the Wilkinson Microwave Anisotropy Probe. This map reveals the small fluctuation seen from the average blackbody temperature of 2.725 kelvin. The red regions are warmer and blue regions are colder by about 0.0002 degrees. (NASA image via Wikimedia Commons.)

You would think that the isotropy of the universe is settled without doubt by the CMBR data. Scientists, however, always look to understand things from more than one perspective. The expansion of the universe from the initial Big Bang has been known since Hubble's law of 1929, and it was discovered in 1998 that this expansion is accelerating, a phenomenon supposedly caused by a mysterious dark energy that comprises about 70% of the mass-energy of the universe.

In 2019, a team of astronomers from the Institut d'Astrophysique de Paris (Paris, France), the Niels Bohr Institute of the University of Copenhagen (Copenhagen, Denmark), and the University of Oxford (Oxford, UK), decided to examine the isotropy of this acceleration through observations of 740 Type Ia supernovae, which are "standard candles" in cosmology.[2-3] They found that this acceleration is not isotropic, but appears to be aligned in the direction of Earth's movement of about 370 km/sec relative to the CMBR with a statistical significance of 3.9σ.[2-3]

Presently, in 2020, another team of astronomers from the Universität Bonn (Bonn, Germany) and the Harvard-Smithsonian Center for Astrophysics (Cambridge, Massachusetts) has examined data for more than 800 galactic clusters from three X-ray telescopes, NASA's Chandra X-ray Observatory, the European Space Agency's XMM-Newton satellite and Japan's Advanced Satellite for Cosmology and Astrophysics, and found a similar anisotropy.[4-6] Says Thomas Reiprich, a professor at the University of Bonn and an author of the study,

"We saw that clusters with the same properties, with similar temperatures, appeared to be less bright than what we would expect in one direction of the sky, and brighter than expected in another direction... The difference was quite significant, around 30 per cent. These differences are not random but have a clear pattern depending on the direction in which we observed in the sky."[5]

The astronomical team examined the very correlated X-ray luminosity vs. temperature relation for the galactic clusters. The measured luminosity is a function of the cosmological distance, while the temperature can be determined without any cosmological assumptions.[4] The analysis revealed two regions where clusters were 30 percent brighter (closer than expected) or fainter (more distant than expected).[6] An anisotropy at about the 5σ level existed such that more intensity was seen towards galactic coordinates (l=303°, b=-27°), which is roughly consistent with the supernovae Ia findings.[4]

Map of cosmic expansion anisotropy.

In this map of the universe, the blue areas appear to expand more slowly than expected, and the yellow areas more rapidly. An isotropic universe would be all red. (Image copyright Konstantinos Nikolaos Migkas, Uni Bonn/Astronomy & Astrophysics. Click for larger image.)

Of course, there are other possible explanations for such an extreme anisotropy, such as undetected regions of gas or dust between us and the galactic clusters, or an affect of dark energy.[5] A phenomenon called bulk flow, caused by gravitational affects of larger, more distant, and unseen clusters, might be a cause. However, most cosmologists don't think that bulk flow occurs over such the large scales probed by the study, roughly five billion light years in extent.[3,6] There's also the idea that the correlation between a galactic cluster’s x-ray temperature and its luminosity are not as precise as thought. Interestingly, the region of greatest apparent cosmic anisotropy is near the place where the Milky Way’s X-ray absorbing gas and dust have their greatest extent.[6]


  1. Latin text of Roger Bacon's Opus Majus at the Boston College Website. This is a PDF facsimile. The quotation reads as follows: "Inter quas sunt principales stellae duodecim signorum, per quas omnia alia specialiter alterantur. Signa vero sunt, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pisces; quae sic nominantur, quia stellae in coelo habent dispositionem rerum sic nominatarum."
  2. Jacques Colin, Roya Mohayaee, Mohamed Rameez, and Subir Sarkar, "Evidence for anisotropy of cosmic acceleration," Astronomy & Astrophysics, vol. 631, no. L13 (November, 2019), https://doi.org/10.1051/0004-6361/201936373. This is an open access paper with a PDF file here. Also appears at arXiv.
  3. Evidence for anisotropy of cosmic acceleration, University of Oxford Press Release, November 20, 2019.
  4. K. Migkas, G. Schellenberger, T. H. Reiprich, F. Pacaud, M. E. Ramos-Ceja and L. Lovisari, "Probing cosmic isotropy with a new X-ray galaxy cluster sample through the LX-T scaling relation," Astronomy & Astrophysics, vol. 636, no. A15 (April, 2020), https://doi.org/10.1051/0004-6361/201936602. This is an open access paper with a PDF file here.
  5. Rethinking cosmology: Universe expansion may not be uniform, European Space Agency Press Release, May 8, 2020.
  6. Lee Billings, "A new study of galaxy clusters suggests the cosmos may not be the same in all directions," Scientific American, April 15, 2020.

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