Scientists studying the deepest picture of the Universe, taken by the Hubble Space Telescope, have been left with a big poser: where are all the stars?
Buried in the image are objects that shone not long after the Big Bang
The Ultra Deep Field is a view of one patch of sky built from 800 exposures.
The picture shows faint galaxies whose stars were shining just a few hundred million years after the Big Bang.
But the image's revelation that fewer stars than expected were being born at this time brings into question current ideas on cosmic evolution.
"Our results based on the Ultra Deep Field are very intriguing and quite a puzzle," says Dr Andrew Bunker, of Exeter University, UK, who led a team studying the new data.
"They're certainly not what I expected, nor what most of the theorists in astrophysics expected."
He is now urging the US space agency (Nasa) to proceed with a servicing mission to upgrade the orbital telescope so it can solve the mystery.
A 'fried' Universe
At issue is the timing of key events in the earliest stages of the Universe.
Scientists believe the super-hot conditions that existed after the Big Bang eventually cooled sufficiently to allow protons, neutrons and electrons to form neutral atoms of hydrogen and helium.
The transition also saw the cosmos plunge into darkness - the stars that could provide the light had yet to ignite.
HUBBLE'S ULTRA DEEP FIELD
It is centred on the constellation Fornax, next to the constellation Orion
The image contains an estimated 10,000 galaxies
It was taken using the ACS and Nicmos instruments
Hubble needed 400 orbits to build the Ultra Deep Field observation
Total time amounted to 11.3 days of continuous viewing
The most distant light was detected at a rate of 1 photon per minute
When they did, from infalling clouds of hydrogen and helium, the "dark ages" gave way to what has been dubbed the "cosmic renaissance".
What is more, these hot, young stars produced intense ultraviolet radiation which "fried" the gas in the Universe - to produce the diffuse intergalactic plasma detectable today.
But the Hubble Ultra Deep Field presents a problem for this story.
When Bunker and colleagues measured the rate of star formation in the image's earliest galaxies, they found it was insufficient to create the levels of radiation needed to produce the intergalactic plasma.
"There is not enough activity to explain the re-ionisation of the Universe," Dr Bunker told the BBC. "Perhaps there was more action in terms of star formation even earlier in the history of the Universe - that's one possibility.
"Another exciting possibility is that physics was very different in the early Universe; our understanding of the recipe stars obey when they form is flawed."
The Hubble data was supported by observations with the Keck telescope in Hawaii and the Gemini telescope in Chile.
It has to be said, the Bunker assessment is not totally shared by all groups working in this area. Four other teams investigating the UDF data have put their own very different interpretations on what they see in the historic image.
For example, the team headed by Dr Massimo Stiavelli, from the Space Telescope Science Institute, in Baltimore, US, believes the populations seen may well have been able to re-ionise the Universe, provided the stars were bigger and possessed much fewer heavier elements than those we see today.
But what all astronomers believe is that to solve this puzzle, they need enhanced space-borne detectors that can better describe the long-wavelength light seen in the most distant stars.
The Hubble telescope will get this capability if Nasa goes ahead with a servicing mission and installs an instrument known as the infrared WideField Camera 3.
This is by no means certain, however, and astronomers may have to wait for the launch of Hubble's successor, the James Webb Telescope, early in the next decade.
"For the first time, we at last have real data to address this final frontier - but we need more observations," said Dr Richard Ellis, of the California Institute of Technology, US, who is passionate in his support of a mission to upgrade Hubble.