Archivi tag: expansion of the universe

‘Mirage’ quintessence and phantom dark energy

Quintessenza e campi “fantasma” sono due tra le varie ipotesi formulate in seguito ai dati ottenuti dai satelliti, come WMAP e Planck, che tentano di spiegare la natura dell’enigmatica energia scura. Oggi, alcuni ricercatori di Barcellona e Atene suggeriscono che entrambe le possibilità sono una sorta di “miraggio” nelle osservazioni e potrebbe essere in definitiva l’energia del vuoto quantistico la principale e l’unica responsabile a celarsi dietro tutto ciò che muove il cosmo.

Cosmologists believe that some three quarters of the Universe are made up of a mysterious dark energy which would explain its accelerated expansion. The truth is that they do not know what it could be, therefore they put forward possible solutions. One is the existence of quintessence, an invisible gravitating agent that instead of attracting, repels and accelerates the expansion of the cosmos. From the Classical World until the Middle Ages, this term has referred to the ether or fifth element of nature, together with earth, fire, water and air. Another possibility is the presence of an energy or phantom field whose density increases with time, causing an exponential cosmic acceleration.

This would reach such speed that it could break the nuclear forces in the atoms and end the Universe in some 20,000 million years, in what is called the Big Rip.

The experimental data that underlie these two hypotheses comes from satellites such as Planck of the European Space Agency (ESA) and Wilkinson Microwave Anisotropy Probe (WMAP) of NASA. Observations from the two probes are essential for solving the so-called equation of the state of dark energy, a characterising mathematical formula, the same as that possessed by solid, liquid and gaseous states. Now researchers from the University of Barcelona (Spain) and the Academy of Athens (Greece) have used the same satellite data to demonstrate that the behaviour of dark energy does not need to resort to either quintessence or phantom energy in order to be explained. The details have been published in the ‘Monthly Notices of the Royal Astronomical Society’ journal. “Our theoretical study demonstrates that the equation of the state of dark energy can simulate a quintessence field, or even a phantom field, without being one in reality, thus when we see these effects in the observations from WMAP, Planck and other instruments, what we are seeing is an mirage”, told SINC Joan Solà, one of the authors from University of Barcelona. “What we think is happening is a dynamic effect of the quantum vacuum, a parameter that we can calculate”, explained the researcher. The concept of the quantum vacuum has nothing to do with the classic notion of absolute nothingness. “Nothing is more ‘full’ than the quantum vacuum since it is full of fluctuations that contribute fundamentally to the values that we observe and measure”, Solà pointed out.

These scientists propose that dark energy is a type of dynamical quantum vacuum energy that acts in the accelerated expansion of our Universe.

This is in contrast to the traditional static vacuum energy or cosmological constant. The drawback with this strange vacuum is that it is the source of problems such as the cosmological constant, a discrepancy between the theoretical data and the predictions of the quantum theory that drives physicists mad. “However, quintessence and phantom fields are still more problematic, therefore the explanation based on the dynamic quantum vacuum could be the more simple and natural one”, concluded Solà.

FECYT/Sinc: Dark energy hides behind phantom fields
arXiv: Effective equation of state for running vacuum: "mirage" quintessence and phantom dark energy
arXiv: Dark energy from a quintessence (phantom) field rolling near potential minimum (maximum)
arXiv: Cosmological constant and vacuum energy: old and new ideas
arXiv: Vacuum energy and cosmological evolution

Per approfondire questo ed altri argomenti: Idee sull’Universo

Exploring the properties of the Universe by Doppler lensing

Sappiamo che l’Universo contiene centinaia di miliardi di galassie, basti guardare le spettacolari immagini che ci ha fornito il telescopio spaziale Hubble. Ce ne sono tante di diverse forme e dimensioni, ma quali sono in definitiva quelle più grandi? E poi, quali sono quelle più vicine alla nostra galassia che sembrano apparentemente più grandi delle altre? Naturalmente, non è possibile rispondere a queste domande analizzando semplicemente le immagini astronomiche poichè, di fatto, è necessario conoscere le distanze a cui si trovano le galassie in modo tale da ricavare una stima delle loro dimensioni reali.

Astronomers have their ways to measure a distance to a galaxy which allows them to solve this conundrum. One of the most popular methods, and in most cases, the only method that can be used to measure a distance to a remote galaxy, is to analyse its electromagnetic spectrum which includes the visible light that enables us to see it. Since the Universe is expanding, all distant galaxies are moving away from us. Because of this motion the spectrum of a galaxy is shifting towards its red part, the redshift as it is known to astronomers. The redshift phenomenon is a manifestation of the Doppler effect, the faster the motion, the larger the shift of the frequency. Therefore, the larger the redshift, the greater the distance to the observed galaxy. The exact relation between the redshift and distance follows from the cosmological model of the Universe. So if astronomers can measure a distance in some other way, then by comparing the observed distance and redshift with a prediction, they can measure the properties of our Universe such as for example the amount of dark matter and dark energy. There is, however, one problem here.

If a galaxy is moving on the top of the global expansion of the Universe, then this motion, via the Doppler effect, contributes to the observed redshift. And galaxies move all the time, just as molecules of the air, or bees within a swarm. The contribution from this local motion is not big if compared to a motion that follows from the expansion of the Universe. Still this additional redshift introduces noise to our measurements. This noise then distorts our estimation of the distance, and therefore our estimation of the real size of the observed galaxy. This is what is called the Doppler lensing, “Doppler” because of the Doppler effect involved, and “lensing” because this effect distorts the inferred size, just as the observed size of an object is distorted when observed through an optical lens. How then can we tell what is the real size of a galaxy? If all galaxies are moving and if their motion distorts our measurements then that sounds like a real mess. However, this “mess” or to be precise the amount of “messiness” can give us a very good insight into what our Universe is made of. Astronomers are now in a situation similar to radar operators who during World War II complained about “noise” in returned echoes due to rain, snow, and sleet. Back then it was a nuisance, now we actually look for this “noise” in order to predict weather. Similarly, if astronomers could measure apparent sizes of a very large number of galaxies, and correlations between them, then they could estimate an average amplitude of the “noise”. Using the technique based on the Doppler lensing effect, they can measure properties of our Universe and estimate how much dark matter and dark energy it contains.

With large galaxy surveys such as Dark Energy Survey (DES) and the contribution from the Australian OzDES we will be able to measure this effect. Further, much larger surveys will follow after completion of the Square Kilometre Array (SKA) telescope, currently being built partly in Western Australia and partly in South Africa, and utilise the Doppler lensing effect to get a better insight into properties and mysteries of our Universe. The calculations and the method itself were recently developed by a group of astronomers from Australia, South Africa, and United Kingdom. The method shows how by measuring correlations in the distortion of sizes of galaxies we can learn about the properties of our Universe (such as amount of dark matter and dark energy). This method and predictions that follow from this method will be presented today at the 8th Workshop of the Australian National Institute for Theoretical Astrophysics (ANITA) hosted by the Sydney Institute for Astronomy (SIfA) at the University of Sydney.

The Conversation: The measure of the universe through doppler lensing

arXiv: Cosmology with Doppler Lensing

Is the Universe really expanding?

Un cosmologo dell’Università di Heidelberg, Christof Wetterich, ritiene che il modello relativo all’espansione dell’Universo, ossia il modello cosmologico standard, potrebbe essere errato. Egli suggerisce che il redshift, cioè lo spostamento verso il rosso misurato dagli astronomi, sarebbe dovuto ad un incremento della massa presente nell’Universo.

For nearly a century, the consensus among astrophysicists has been that the Universe started with a Big Bang and has been expanding ever since. This hypothesis formed because researchers found that in analyzing the light emitted from stars, a redshift occurred, where its frequency changes as an object that emits light moves away from us.

But Wetterich says the redshift might me due to something else, an increase in the total mass in the universe.

Wetterich’s idea is that light emitted from an atom is governed by the mass of its particles, if that atom were to become larger in mass, the light that it emits would change in frequency as its electrons became more energetic. More energy would appear as light moving toward the blue spectrum, while less energy (an atom losing mass), would move toward the red spectrum. Thus, Wetterich reasons, if the mass of observable objects were once less, we would now see them with a redshift as they expand.

If his line of reasoning is true, Wetterich says it’s possible that the Universe is actually contracting.

Wetterich’s paper hasn’t been peer reviewed yet, but thus far, comments by others in the field suggest openness to this new line of thinking. That might be because one exciting prospect of this new theory is that it would do away with the idea of a singularity existing just before the Big Bang, a point at which conventional physics breaks down. Instead it might suggest that the Universe is simply in a constant state of flux with no real beginning and no real end. Unfortunately, Wetterich’s theory can’t be tested because of the relative nature of mass. Everything we are able to see has a mass that is relative in size to everything else. Thus if it’s all growing, we wouldn’t have anything to measure it against to see that it’s happening.

Nature: Cosmologist claims Universe may not be expanding
arXiv: A Universe without expansion

Measuring the Universe More Accurately Than Ever Before

After nearly a decade of careful observations an international team of astronomers has measured the distance to our neighbouring galaxy, the Large Magellanic Cloud, more accurately than ever before. This new measurement also improves our knowledge of the rate of expansion of the Universe, the Hubble constant, and is a crucial step towards understanding the nature of the mysterious dark energy that is causing the expansion to accelerate. The team used telescopes at ESO’s La Silla Observatory in Chile as well as others around the globe. These results appear in the 7 March 2013 issue of the journal Nature.

Astronomers survey the scale of the Universe by first measuring the distances to close-by objects and then using them as standard candles to pin down distances further and further out into the cosmos. But this chain is only as accurate as its weakest link. Up to now finding an accurate distance to the Large Magellanic Cloud (LMC), one of the nearest galaxies to the Milky Way, has proved elusive. As stars in this galaxy are used to fix the distance scale for more remote galaxies, it is crucially important. But careful observations of a rare class of double star have now allowed a team of astronomers to deduce a much more precise value for the LMC distance: 163 000 light-years. “I am very excited because astronomers have been trying for a hundred years to accurately measure the distance to the Large Magellanic Cloud, and it has proved to be extremely difficult,” says Wolfgang Gieren (Universidad de Concepción, Chile) and one of the leaders of the team. “Now we have solved this problem by demonstrably having a result accurate to 2%.” The improvement in the measurement of the distance to the Large Magellanic Cloud also gives better distances for many Cepheid variable stars. These bright pulsating stars are used as standard candles to measure distances out to more remote galaxies and to determine the expansion rate of the Universe, the Hubble constant. This in turn is the basis for surveying the Universe out to the most distant galaxies that can be seen with current telescopes. So the more accurate distance to the Large Magellanic Cloud immediately reduces the inaccuracy in current measurements of cosmological distances. The astronomers worked out the distance to the Large Magellanic Cloud by observing rare close pairs of stars, known as eclipsing binaries. As these stars orbit each other they pass in front of each other. When this happens, as seen from Earth, the total brightness drops, both when one star passes in front of the other and, by a different amount, when it passes behind. By tracking these changes in brightness very carefully, and also measuring the stars’ orbital speeds, it is possible to work out how big the stars are, their masses and other information about their orbits. When this is combined with careful measurements of the total brightness and colours of the stars remarkably accurate distances can be found. This method has been used before, but with hot stars. However, certain assumptions have to be made in this case and such distances are not as accurate as is desirable. But now, for the first time, eight extremely rare eclipsing binaries where both stars are cooler red giant stars have been identified. These stars have been studied very carefully and yield much more accurate distance values — accurate to about 2%. “ESO provided the perfect suite of telescopes and instruments for the observations needed for this project: HARPS for extremely accurate radial velocities of relatively faint stars, and SOFI for precise measurements of how bright the stars appeared in the infrared,” adds Grzegorz Pietrzyński (Universidad de Concepción, Chile and Warsaw University Observatory, Poland), lead author of the new paper in Nature. “We are working to improve our method still further and hope to have a 1% LMC distance in a very few years from now. This has far-reaching consequences not only for cosmology, but for many fields of astrophysics,” concludes Dariusz Graczyk, the second author on the new Nature paper.

ESO: Measuring the Universe More Accurately Than Ever Before
Research paper in Nature: An eclipsing binary distance to the Large Magellanic Cloud accurate to 2 per cent