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8. The First Stars Formed Later Than Previously Thought
John Austin

The cosmic microwave background. This shows a snapshot of the earliest light imprinted on the sky when the universe was only 380,000 years old. The colours indicate small differences in temperature, with the difference between hot and cold regions of about 0.0002 K. Credit: ESA and the Planck Collaboration.

Measurements of the cosmic microwave background now reveal that the first stars formed later than previously thought, consistent with observations from the Hubble Space Telescope.

Formation of Stars in the Universe

Following the big bang when the universe formed, energy and matter started to clump together. This material was mainly hydrogen which became gradually compressed by gravitational forces. Once the density of material locally reached critical levels it became enough to produce nuclear fusion reactions (which also powers our sun). This denotes the time at which the stars started to shine. Until then there would have been a period of lower intensity radiation, known as the "dark ages".

A schematic diagram of the history of the universe from the big bang to the present day. Credit: ESA and the Planck Collaboration.

The big bang itself created background radiation in the universe which has been measured. This radiation can be thought of as the afterglow of the big bang, and would have been produced about 380,000 years after the big bang. The radiation would have been produced once hydrogen atoms had formed and the temperature of the universe had cooled to a few thousand degrees Celsius. Over the last 13.8 billion years of evolution of the universe that radiation has cooled and is now only detectable as a microwave signal equivalent to a temperature of about 2.725 K (degrees above absolute zero) or -270.4 oC. By observing the unevenness in the background radiation observed by the Wilkinson Microwave Anisotropy Probe, scientists previously estimated that the stars started to shine about 420 million years after the big bang.

Observations from the Hubble Space telescope

The most distant galaxy ever seen in the universe Credit: NASA, ESA, G. Illingworth (University of California, Santa Cruz), R. Bouwens (University of California, Santa Cruz and Leiden University), and the HUDF09 Team.

Using observations from the Ultra Deep Field of the Hubble Space Telescope objects can be perceived a considerable distance away. Because of the finite speed of light (299,792,458 m/s --- now the definition of the metre) looking at distant objects is equivalent to looking into the past. Using the Ultra Deep Field of the Hubble, exposures were taken in 2009 and 2010, requiring a total of 8 days of observing. The faint red blob, 500 million times fainter than the faintest stars visible by the naked eye, is barely perceptible in the main image. Based on the red shift of the object it is calculated to be 13.2 billion years old. Red shift is a measure of the change in wavelength of light due to the motion of an object away from us, the astronomical equivalent of the Doppler effect. Because of the expansion of the universe, red shift is a direct measurement of the distance of an object.

With the current estimates of the age of the universe placed at 13.8 billion years this would place the object at about 500-600 million years after the big bang. This would mean that the earliest stars should have formed about 100 million years or more after the measurements of the background radiation would indicate.

Resolution of the Timing Dilemma

While it seems small in relation to the total age of the universe, the 100 million year gap has now been bridged by improved measurements of the background radiation from the European Planck Satellite. Observations released on 5 February 2015 now indicate that the first stars shone about 560 million years after the big bang. As noted by Prof George Efstathiou, one of the leaders of the Planck Science Collaboration, the difference of 140 million years might not seem that significant in the context of the 13.8-billion-year history of the cosmos, but it is a large change in our understanding of how certain key events progressed at the earliest times. It also places the results firmly in agreement with the Hubble observations.

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Article initially prepared 6 February 2015.

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Website revised by John Austin, 6/2/2015. © Enigma Scientific Publishing, 2015.