Isotropy and homogeneity are the two pillars of the cosmological principle. Isotropy – the same in all directions, and homogeneity – the same at all locations, provide us with a basis for understanding our universe and how it operates on the largest scales. According to this principle, no one point in the universe is more preferred over another. Our place in the universe is not a privileged one [Liddle & Loveday 2008]. The physical laws we know here on Earth are the same laws observed and followed everywhere else in the universe. However, these two elements only become apparent at scales greater than approximately 300 Mpc [Yadav 2010], where we begin to see a relatively consistent distribution of stars, galaxies, and matter. It is important to remember that the cosmological principle is more a working hypothesis and not a true scientific principle [Holcomb & Hawley 2005]. Still, it forms the very foundation of modern cosmology, without which it would be difficult, if not impossible, to make any conclusions regarding the nature of our universe.
A History of the Cosmological Principle
The concept of the cosmological principle was first asserted in Isaac Newton’s Philosophiæ Naturalis Principia Mathematica (1687). In a break from the geocentric Ptolemaic view, Newton saw the earth as a sphere orbiting the sun within an empty space that extended in all directions to large distances. He also stated that: 1) all of the bodies in the solar system were composed of similar matter; 2) that the sun and the distant stars were of the same nature; and, 3) that the physical laws of motion on Earth were valid at great distances beyond Earth [Harrison 2000].
Albert Einstein would further codify the principle in 1917 with the words “no average property of the cosmic medium defines a preferred place or a preferred direction in space [Einstein 1917].” It would be Edward Milne, though, in 1933, who would officially name it the cosmological principle [Harrison 2000].
Once the premise of a universe that was the same in all locations and in all directions was generally accepted, it would only be a short time before the concept of a universe that was the same at all times was added. This would become known as the perfect cosmological principle. Any universe that obeys the perfect cosmological principle must appear to be the same, on average, everywhere and for all times. This form of the principle was proposed by Bondi, Gold, and Hoyle in 1948. This view of the universe being the same in all locations, in all directions, and in all times would become known as the steady state theory [Harrison 2000]. It vied with the Big Bang model for almost twenty years, until the discovery of the cosmic background radiation in 1965. This last remnant of the Big Bang, coupled with the observation that radio galaxies were more prevalent at earlier stages of the universe’s development, finally dispelled the steady state theory [Whittle 2008].
Further observational evidence has accumulated over the years, strengthening both the cosmological principle and the Big Bang model. For example, the 2dF QSO Redshift Survey at the Anglo-Australian Telescope, conducted from 1997 until 2002, mapped over 23,000 quasars and their redshifts [Croom 2004]. (See Fig. 1) The distribution shows a homogeneous pattern, as well as greater numbers of quasars at larger redshifts, as would be expected in a non-steady state universe.
The distribution of all quasars found in the 2dF QSO Redshift Survey at the Anglo-Australian Telescope. Earth is located at the center of the plot, with quasars at increasingly large distances moving away from the center. (http://www.gemini.edu/project/announcements/press/2004-11.html#media)
Current Status of the Cosmological Principle
Despite such observational evidence for the validity of the cosmological principle, nagging questions are being raised. Recent observational data calls into question the concepts of both homogeneity and isotropy, without which there can be no cosmological principle as we know it today. The recent discoveries of hierarchal superstructures which violate the homogeneity scale are particularly troublesome. (See Table 1) Two particular cases stand out.
In November 2012, Clowes et al. published a report studying quasars in the DR7QSO catalogue of the Sloan Digital Sky Survey (SDSS). Their research uncovered one of the largest structures yet observed at that time in the universe: a large quasar group (LQG) with more than 73 quasars and a size of approximately 500 Mpc [Clowes 2012]. Designated U1.27, it was the largest structure discovered in the universe at that time.
An even greater challenge to homogeneity, though, occurred in November 2013, when Horvath et al. published their study of the distribution of gamma-ray bursts (GRB) in the constellations Hercules and Corona Borealis. Their sample of 273 GRB revealed a structure of immense size: a galactic superstructure measuring over 10 billion light-years across [Horvath 2013]. Named the Hercules–Corona Borealis Great Wall, it poses a serious problem for the cosmological principle: the currently accepted homogeneity scale is approximately 300 – 370 Mpc [Yadav 2010]. As such, we should not see structures that exceed this scale.
Homogeneity, though, is not the only aspect of the principle now being questioned. In 2011, researchers Rong-Gen Cai and Zhong-Liang Tuo published the results of their study of Type Ia supernovae (SNIa). Through hemispheric comparison of SNIa data, they were able to show that the universe’s expansion seems to be accelerating faster in the direction of a small part of the northern galactic hemisphere [Cai 2011]. Such a preferred direction would violate the concept of isotropy; a serious blow to the cosmological principle.
Exceedingly large structures which violate the homogeneity scale, such as the Hercules–Corona Borealis Great Wall and Large Quasar Groups, pose a direct challenge to the concept of homogeneity in the cosmological principle. Additionally, should further tests and observations confirm the initial findings of a possible preferred direction in the space-time expansion, the concept of isotropy would be threatened.
Further observations and testing are essential in order to either confirm or refute these findings definitively. The cosmological principle is the very foundation upon which modern cosmology is built. At the very least, a refined definition of the cosmological principle must be developed as new observational data becomes available.
All web addresses cited in the body of this work refer to versions accessed as of 2014 February 23.
[Cai 2011] Cai, Rong-Gen, Tuo, Zhong-Liang, Direction Dependence of the Deceleration Parameter, arXiv:1109.0941 [astro-ph.CO], 10.1088/1475-7516/2012/02/004.
JCAP 1202 (2012) 004.
[Clowes 2012] Clowes, R., Harris, K., Raghunathan, S., Campusano, L., Soechting, I., Graham, M., A Structure in the Early Universe at Z ~ 1.3 That Exceeds the Homogeneity Scale of the R-W Concordance Cosmology, Nov. 2012, Accepted for publication in Monthly Notices of the Royal Astronomical Society. 9 pages, 3 figures, arXiv:1211.6256 [astro-ph.CO]
[Croom 2004] Croom, S., Schade, D., Boyle, B., Shanks, T., Miller, L., Smith, R., Gemini Imaging of QSO Host Galaxies at z ~2, Astrophys.J.606:126-138, 2004, DOI:10.1086/382747, arXiv:astro-ph/0401442
[Einstein 1917] Einstein, Albert. Cosmological Considerations of the General Theory of Relativity, Sitzungsber.Preuss.Akad.Wiss.Berlin (Math.Phys.) 1917 (1917) pp.142-152.
[Harrison 2000] Harrison, Edward. Cosmology: The Science of the Universe, 2nd Edition, Cambridge University Press, UK, 2000.
[Hawley & Holcomb 2005] Hawley, John F.; Holcomb, Katherine A. Foundations of Modern Cosmology, 2nd Edition, Oxford University Press, UK, 2005.
[Horvath 2013] Horvath, I., Hakkila, J., Bagoly, Z., The Largest Structure of the Universe, Defined by Gamma-Ray Bursts, 7th Huntsville Gamma-Ray Burst Symposium, GRB 2013: paper 33 in eConf Proceedings C1304143, arXiv:1311.1104 [astro-ph.CO]
[Liddle & Loveday 2008] Liddle, Andrew; Loveday, Jon. Oxford Companion to Cosmology, 2nd Edition, Oxford University Press, UK, 2008.
[Whittle 2008] Whittle, Mark. Cosmology: The History and Nature of Our Universe, The Great Courses, Virginia, USA, 2008.
[Yadav 2010] Yadav, J.J., Bagla, J.S., Khandai, N., Fractal Dimension as a Measure of the Scale of Homogeneity, Mon.Not.Roy.Astron.Soc.405:2009, 2010, DOI:10.1111/j.1365-2966.2010.16612.x, arXiv:1001.0617 [astro-ph.CO]