Physicists from Rio de Janeiro, with colleagues abroad, are mastering the art of using a special phenomenon that affects light to understand the composition and structure of the Universe on a large scale. Called a gravitational lens, this phenomenon serves as a kind of cosmic magnifying glass and allows us to see celestial objects that would often not be visible because they are too far away. With the help of gravitational lenses, Martín Makler, Bruno Moraes and Aldée Charbonnier of the Brazilian Center for Physics Research (CBPF) have developed a new map of dark matter, one of the most abundant and mysterious components of the cosmos. “This is the most extensive survey of its kind made with good quality images for a contiguous area of the sky,” says Makler, coordinator of the Brazilian contribution to the project, conducted in partnership with Chinese, European, and Canadian groups.
The researchers used a telescope in Hawaii to examine 16 million stars and galaxies in a strip of the celestial equator, initially. This area of the sky is heavily studied since it is visible from both Earth’s northern and southern hemispheres. Based on data from the nearly 3 million objects for which they obtained detailed information, they created a two-dimensional map of the distribution of dark matter in a band of the celestial equator about 7 billion light-years from Earth. The amount of dark matter in the galaxies in this region of the sky is five to six times greater than the amount of normal matter, such as that which makes up stars, planets and living beings. “About 80% of matter in the structures that act as gravitational lenses is dark matter,” explains Makler. This ratio is consistent with that observed in other large-scale studies of the Universe.
According to the researchers, this result is compatible with the predictions of the model most accepted today to explain the behavior of the Universe since the Big Bang, 13.8 billion years ago through today: the lambda cold dark matter, or lambda CDM. This model proposes that just under 70% of everything (energy and matter) that makes up the Universe corresponds to what is called dark energy, an unknown form of energy that seems to be latent in empty space. About a quarter of the cosmos is believed to be composed of dark matter, formed by particles that neither emit nor absorb light, while the remaining 5% is composed of normal matter.
“It may not seem like much to corroborate the model currently accepted as the standard,” says Makler, “but you have to take into account that this is a prelude of what’s to come, since new projects will examine much larger areas of the sky with an image quality similar to ours.” The CBPF researcher says that, for the region of sky now mapped, there had been no high-resolution, detailed survey. “That’s what we decided to do: the largest survey of a contiguous area of the sky at this resolution,” he says.
From a technical standpoint, it was a considerable challenge. Since it does not interact with light, but only with ordinary matter through gravity, the presence of dark matter was inferred based on the distortion it causes in the path of the light reaching us from distant galaxies. This effect, called gravitational lensing, varies in intensity according to the distribution of mass between the galaxies viewed and observers on Earth.
Gravitational lenses are among the many effects predicted by the theory of general relativity, formulated by the German physicist Albert Einstein (1879-1955). Einstein changed how we understand gravity when he showed that space and time are malleable—especially under the action of massive objects such as stars, galaxies or clusters of galaxies. Centuries earlier, Isaac Newton had explained gravity as an attractive force that a body of given mass exerts on another. That which was understood as gravitational attraction based on Newton’s ideas, and explained the Earth rotating around the Sun, changed after Einstein. Now it is understood to be the result of curvature of space due to the mass of the sun, as if the star were a bowling ball placed on a pillow. If space is distorted by massive objects, this means that light also bends when passing through this region of space. One of the consequences of the change in the path of light passing through this region is that the image of a more distant object may be enlarged (see infographic).
In practice, much more complicated things can happen. “The image can also be duplicated, stretched and distorted, among other phenomena,” explains physicist Miguel Quartin, of the Federal University of Rio de Janeiro (UFRJ), who has been using the gravitational lens effect to study supernovae, stellar explosions that serve as one of the principal “rulers” used to measure the accelerated expansion of the Universe.
Even gravitational lenses may have different intensities and be strong or weak. The strong version of the phenomenon—in which there is, for example, strong distortion and magnification of the image—only happens if the object that acts as the lens has a very large mass and there is very good alignment between it, the observer on Earth and the source of the light (the distant galaxy whose image is distorted). When a weak version occurs, one must take into account much slighter distortions generated by the sum of the gravitational influence of objects near the region being observed. “In this case, the distortion may be very subtle,” says Makler, of the CBPF.
If the effects of gravitational lensing seen by the CBPF physicists represent the possibility of reducing something of the aura of mystery surrounding dark matter, the studies conducted by Miguel Quartin’s team at UFRJ are more directly related to an even more enigmatic component of the cosmos, called dark energy. One of the principal tools used to measure the intensity of dark energy, which acts as a type of antigravity, repelling objects and causing the Universe to expand at an accelerated rate, is type Ia supernovae.
The brightness of a thousand suns
At UFRJ, Quartin, Tiago Castro and Valerio Marra, in collaboration with a researcher in Germany, recently showed that the gravitational lensing effect must be taken into account when studying these supernovae.
Although they arise from the remains of small stars, called white dwarfs—and the probable fate of the Sun 7 billion years from now—supernovae release prodigious amounts of energy. The overwhelming brilliance emitted in the explosion of a supernovae acts as an excellent standard candle and serves as a cosmic bookmark. This is because Ia supernovae emit light of known and stable intensity. It is known, furthermore, that the apparent intensity of a light source decreases as the distance between the source and the observer increases. In fact, the apparent intensity of a light source decreases in proportion to the inverse square of the distance between the source and the observer. This allows us to calculate the distance between the light and the observer. When mapping the presence of this type of supernova throughout the Universe, it is as if astronomers were looking at a succession of light poles along a road, with weaker lights indicating the farthest points.
In the case of the cosmos, however, the road is getting longer. This becomes clearer when one thinks of another phenomenon, called red shift, which is the optical equivalent of the distortion of the sound of a siren when an ambulance is rapidly moving away from you. Just as sound waves are distorted by motion, the wavelength of light waves emitted by a star that is moving away from the Earth also changes from the perspective of the observer, moving closer to the wavelength of red light. The comparison between the distance inferred by means of standard candles and the red shift is one of the principal clues that the universe is expanding rapidly, driven by dark energy.
Quartin reminds us, however, that the apparently simple rule for estimating the intensity of light from supernovae is rarely sufficient to be able to use these objects as good measures of cosmic expansion. The problems that interfere with the observed intensity of the supernovae range from mundane things like clouds of interstellar dust between the supernova and the Earth, making the object appear less bright than it is, to distortions in light intensity caused by gravitational lensing.
In recent work, Quartin and his colleagues used models of the distribution of matter in the cosmos to correct for the interference of these phenomena, including gravitational lenses and the light from supernovae. “The idea was also to take one step further and see if, with supernovae and gravitational lenses, we could understand something more about the structure of the Universe and how matter is distributed in it,” says Quartin.
Applying this proposal to nearly 700 Ia supernovae—some of which emitted the light observed now nearly 8 billion years ago—the team from UFRJ found that this is, indeed, possible. “It is the first result of a new technique, which agrees with what is known from other methods,” says Quartin. “The important thing is that the technique passed the test.”
He points out that, over the next 10 years, data will be available for this type of estimate from about 100,000 supernovae instead of just 700. Then we will be able to reach high levels of accuracy and find out if the information derived from this methodology changes what we know about dark matter and dark energy. “If the methodologies agree, great; if they disagree, it will be because some hypothesis is wrong and we will be able to find this inconsistency by analyzing the same phenomenon with different techniques,” he explains. “This is how science advances.” The hope is that advances of this type provide clues as to the nature of these two components of the cosmos which, for now, can only be investigated based on the effects they produce.
SHAN, H. et al. Weak lensing mass map and peak statistics in Canada-France-Hawaii Telescope Stripe 82 survey. Monthly Notices of the Royal Astronomical Society. v. 442. jun. 2014.
LI, R. et al. First galaxy-galaxy lensing measurement of satellite halo mass in the CFHT Stripe 82 Survey. Monthly Notices of the Royal Astronomical Society. v. 437. jan. 2014.
MARRA, V.; QUARTIN, M.; AMENDOLA, L. Accurate weak lensing of standard candles. I. Flexible cosmological fits. Physical Review D. 5 set. 2013.
QUARTIN, M.; MARRA, V.; AMENDOLA, L. Accurate weak lensing of standard candles. II. Measuring 8 with supernovae. Physical Review D. 28 jan. 2014.
CASTRO, T.; QUARTIN, M. First measurement of 8 using supernova magnitudes only.Monthly Notices of the Royal Astronomical Society Letters. 10 jul. 2014.