[/caption]Astronomers have uncovered yet another clue in their quest to understand the Universe’s early life: the most distant quasar ever observed. At a redshift of 7.1, it is a relic from when the cosmos was just 770 million years old – just 5% of its age today.
Quasars are extremely old, outrageously luminous balls of radiation that were prevalent in the early Universe. Each is thought to have been fueled at its core by an incredibly powerful supermassive black hole. The most recent discovery (which carries the romantic name ULAS J1120+0641) is noteworthy for a couple of reasons. First of all, its supermassive black hole weighs approximately two billion solar masses – an impressive feat of gravity so soon after the Big Bang. It is also incredibly bright, given its great distance. “Objects that lie at such large distance are almost impossible to find in visible-light surveys because their light is stretched by the expansion of the universe,” said Dr. Simon Dye of the University of Nottingham, a member of the team that discovered the object. “This means that by the time their light gets to Earth, most of it ends up in the infrared part of the electromagnetic spectrum.” Due to these effects, only about 100 visible quasars exist in the sky at redshifts higher than 7.
Up until recently, the most distant quasar observed was at a redshift of 6.4; but thanks to this discovery, astronomers can probe 100 million years further into the history of the Universe than ever before. Careful study of ULAS J1120+0641 and its properties will enable scientists to learn more about galaxy formation and supermassive black hole growth in early epochs. The research was published in the June 30 issue of Nature.
The Pleiades, Anglo-Australian Observatory/Royal Observatory
[/caption]
One of the consequences of Einsteins theories of relativity is that everything will be affected by gravitational potentials, regardless of their mass. The effect of this is observed in experiments demonstrating the potential for gravity to bend light. But a more subtle realization is that light escaping such a gravitational well must lose energy, and since energy for light is related to wavelength, this will cause the light to increase in wavelength through a process known as gravitational redshifting.
Since the amount of redshift is dependent on just how deeply inside a gravitational well a photon is when it starts its journey, predictions have shown that photons being emitted from the photosphere of a main sequence star should be more redshifted than those coming from puffed out giants. With resolution having reached the threshold to detect this difference, a new paper has attempted to observationally detect this difference between the two.
Historically, gravitational redshifts have been detected on even more dense objects such as white dwarfs. By examining the average amount of redshifts for white dwarfs against main sequence stars in clusters such as the Hyades and Pleiades, teams have reported finding gravitational redshifts on the order of 30-40 km/s (NOTE: the redshift is expressed in units as if it were a recessional Doppler velocity, although it’s not. It’s just expressed this way for convenience). Even larger observations have been made for neutron stars.
For stars like the Sun, the expected amount of redshift (if the photon were to escape to infinity) is small, a mere 0.636 km/s. But because Earth also lies in the Sun’s gravitational well the amount of redshift if the photon were to escape from the distance of our orbit would only be 0.633 km/s leaving a distance of only ~0.003 km/s, a change swamped by other sources.
Thus, if astronomers wish to study the effects of gravitational redshift on stars of more normal density, other sources will be required. Thus, the team behind the new paper, led by Luca Pasquini from the European Southern Observatory, compared the shift among stars of the middling density of main sequence stars against that of giants. To eliminate effects of varying Doppler velocities, the team chose to study clusters, which have consistent velocities as a whole, but random internal velocities of individual stars. To negate the latter of these, they averaged the results of numerous stars of each type.
The team expected to find a discrepancy of ~0.6 km/s, yet when their results were processed, no such difference was detected. The two populations both showed the recessional velocity of the cluster, centered on 33.75 km/s. So where was the predicted shift?
To explain this, the team turned to models of stars and determined that main sequence stars had a mechanism which could potentially offset the redshift with a blueshift. Namely, convection in the atmosphere of the stars would blueshift material. The team states that low mass stars made up the bulk of the survey due to their number and such stars are thought to undergo greater amounts of convection than most other types of stars. Yet, it is still somewhat suspect that this offset could so precisely counter the gravitational redshift.
Ultimately, the team concludes that, regardless of the effect, the oddities observed here point to a limitation in the methodology. Trying to tease out such small effects with such a diverse population of stars may simply not work. As such, they recommend future investigations target only specific sub-classes for comparison in order to limit such effects.
[/caption]
This time it was the gravitational redshift part of General Relativity; and the stringency? An astonishing better-than-one-part-in-100-million!
How did Steven Chu (US Secretary of Energy, though this work was done while he was at the University of California Berkeley), Holger Müler (Berkeley), and Achim Peters (Humboldt University in Berlin) beat the previous best gravitational redshift test (in 1976, using two atomic clocks – one on the surface of the Earth and the other sent up to an altitude of 10,000 km in a rocket) by a staggering 10,000 times?
By exploited wave-particle duality and superposition within an atom interferometer!
Cesium atom interferometer test of gravitational redshift (Courtesy Nature) About this figure: Schematic of how the atom interferometer operates. The trajectories of the two atoms are plotted as functions of time. The atoms are accelerating due to gravity and the oscillatory lines depict the phase accumulation of the matter waves. Arrows indicate the times of the three laser pulses. (Courtesy: Nature).
Gravitational redshift is an inevitable consequence of the equivalence principle that underlies general relativity. The equivalence principle states that the local effects of gravity are the same as those of being in an accelerated frame of reference. So the downward force felt by someone in a lift could be equally due to an upward acceleration of the lift or to gravity. Pulses of light sent upwards from a clock on the lift floor will be redshifted when the lift is accelerating upwards, meaning that this clock will appear to tick more slowly when its flashes are compared at the ceiling of the lift to another clock. Because there is no way to tell gravity and acceleration apart, the same will hold true in a gravitational field; in other words the greater the gravitational pull experienced by a clock, or the closer it is to a massive body, the more slowly it will tick.
Confirmation of this effect supports the idea that gravity is geometry – a manifestation of spacetime curvature – because the flow of time is no longer constant throughout the universe but varies according to the distribution of massive bodies. Exploring the idea of spacetime curvature is important when distinguishing between different theories of quantum gravity because there are some versions of string theory in which matter can respond to something other than the geometry of spacetime.
Gravitational redshift, however, as a manifestation of local position invariance (the idea that the outcome of any non-gravitational experiment is independent of where and when in the universe it is carried out) is the least well confirmed of the three types of experiment that support the equivalence principle. The other two – the universality of freefall and local Lorentz invariance – have been verified with precisions of 10-13 or better, whereas gravitational redshift had previously been confirmed only to a precision of 7×10-5.
In 1997 Peters used laser trapping techniques developed by Chu to capture cesium atoms and cool them to a few millionths of a degree K (in order to reduce their velocity as much as possible), and then used a vertical laser beam to impart an upward kick to the atoms in order to measure gravitational freefall.
Now, Chu and Müller have re-interpreted the results of that experiment to give a measurement of the gravitational redshift.
In the experiment each of the atoms was exposed to three laser pulses. The first pulse placed the atom into a superposition of two equally probable states – either leaving it alone to decelerate and then fall back down to Earth under gravity’s pull, or giving it an extra kick so that it reached a greater height before descending. A second pulse was then applied at just the right moment so as to push the atom in the second state back faster toward Earth, causing the two superposition states to meet on the way down. At this point the third pulse measured the interference between these two states brought about by the atom’s existence as a wave, the idea being that any difference in gravitational redshift as experienced by the two states existing at difference heights above the Earth’s surface would be manifest as a change in the relative phase of the two states.
The virtue of this approach is the extremely high frequency of a cesium atom’s de Broglie wave – some 3×1025Hz. Although during the 0.3 s of freefall the matter waves on the higher trajectory experienced an elapsed time of just 2×10-20s more than the waves on the lower trajectory did, the enormous frequency of their oscillation, combined with the ability to measure amplitude differences of just one part in 1000, meant that the researchers were able to confirm gravitational redshift to a precision of 7×10-9.
As Müller puts it, “If the time of freefall was extended to the age of the universe – 14 billion years – the time difference between the upper and lower routes would be a mere one thousandth of a second, and the accuracy of the measurement would be 60 ps, the time it takes for light to travel about a centimetre.”
Müller hopes to further improve the precision of the redshift measurements by increasing the distance between the two superposition states of the cesium atoms. The distance achieved in the current research was a mere 0.1 mm, but, he says, by increasing this to 1 m it should be possible to detect gravitational waves, predicted by general relativity but not yet directly observed.
