The Universe is Isotropic

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Peter Lobner, Updated 12 January 2021

The concepts of up and down appear to be relatively local conventions that can be applied at the levels of subatomic particles, planets and galaxies. However, the universe as a whole apparently does not have a preferred direction that would allow the concepts of up and down to be applied at such a grand scale.

A 7 September 2016 article entitled, “It’s official: You’re lost in a directionless universe,” by Adrian Cho, provides an overview of research that demonstrates, with a high level of confidence, that the universe is isotropic. The research was based on data from the Planck space observatory. In this article, Cho notes:

“Now, one team of cosmologists has used the oldest radiation there is, the afterglow of the big bang, or the cosmic microwave background (CMB), to show that the universe is “isotropic,” or the same no matter which way you look: There is no spin axis or any other special direction in space. In fact, they estimate that there is only a one-in-121,000 chance of a preferred direction—the best evidence yet for an isotropic universe. That finding should provide some comfort for cosmologists, whose standard model of the evolution of the universe rests on an assumption of such uniformity.”

The European Space Agency (ESA) developed the Planck space observatory to map the CMB in microwave and infrared frequencies at unprecedented levels of detail. Planck was launched on 14 May 2009 and was placed in a Lissajous orbit around the L2 Lagrange point, which is 1,500,000 km (930,000 miles) directly behind the Earth. L2 is a quiet place, with the Earth shielding Planck from noise from the Sun. The approximate geometry of the Earth-Moon-Sun system and a representative spacecraft trajectory (not Planck, specifically) to the L2 Lagrange point is shown in the following figure.

Lissajous orbit L2Source: Abestrobi / Wikimedia Commons

The Planck space observatory entered service on 3 July 2009. At the end of its service life, Planck departed its valuable position at L2, was placed in a heliocentric orbit, and was deactivated on 23 October 2013. During more than four years in service, Planck performed its CBM mapping mission with much greater resolution than NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), which operated from 2001 to 2010.  Planck was designed to map the CMB with an angular resolution of 5-10 arc minutes and a sensitivity of a millionth of a degree.

One key result of the Planck mission is the all-sky survey shown below.

Planck all-sky survey 2013 CBM temperature map shows anisotropies in the temperature of the CMB at the full resolution obtained by Planck. Source: ESA / Planck Collaboration

ESA characterizes this map as follows:

“The CMB is a snapshot of the oldest light in our Universe, imprinted on the sky when the Universe was just 380,000 years old. It shows tiny temperature fluctuations that correspond to regions of slightly different densities, representing the seeds of all future structure: the stars and galaxies of today.”

The researchers who reported that the universe was isotropic noted that an anisotropic universe would leave telltale patterns in the CMB. However, these researchers found that the actual CMB shows only random noise and no signs of such patterns.

The researchers who reported that the universe was isotropic noted that an anisotropic universe would leave telltale patterns in the CMB.  However, these researchers found that the actual CMB shows only random noise and no signs of such patterns.

In 2015, the ESA / Planck Collaboration used CMB data to estimate the age of the universe at 13.813 ± 0.038 billion years.  This was lightly higher than, but within the uncertainty band of, an estimate derived in 2012 from nine years of data from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft.

In July 2018, the ESA / Planck Collaboration published the “Planck Legacy” release of their results, which included the following two additional CBM sky survey maps.

Planck all-sky survey 2013 CBM smoothed temperature map (top) and smoothed temperature + polarization map (bottom). Source: ESA / Planck Collaboration

The ESA/Planck Collaboration described these two new maps as follows:

  • (In the top map), “the temperature anisotropies have been filtered to show mostly the signal detected on scales around 5º on the sky. The lower view shows the filtered temperature anisotropies with an added indication of the direction of the polarized fraction of the CMB.”
  • “A small fraction of the CMB is polarized – it vibrates in a preferred direction. This is a result of the last encounter of this light with electrons, just before starting its cosmic journey. For this reason, the polarization of the CMB retains information about the distribution of matter in the early Universe, and its pattern on the sky follows that of the tiny fluctuations observed in the temperature of the CMB” (in the 2013 map, above).

Using Planck CMB data, the ESA / Planck Collaboration team has estimated the value of the Hubble constant. Their latest estimate, in 2018, was 67.4 km / second / megaparsec with an uncertainty of less than 1%.  This is lower than the value derived from astrophysical measurements: 73.5 km / second / megaparsec with an uncertainty of 2%.

You’ll find more details on the Planck mission and scientific results on the ESA’s website at the following link: http://www.esa.int/Our_Activities/Space_Science/Planck

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