Did LIGO really see massive black holes?

GW being emitted by a pair of black holes

The LIGO (and now Virgo) experiment has opened a new window to explore one of the most mysterious objects in nature, black holes (BH). When two black holes merge, they create a cataclysmic event that sends waves through the fabric of space itself and can travel cosmic distances. This is similar to an earthquake shaking Earth. These waves are known as gravitational waves (GW) and until 2015 they were just pure speculation as no experiment was ever able to detect them. Despite the tremendous amount of energy released when two BH merge (a binary BH merger), these waves, or ripples in space-time, are incredibly difficult to observe. The distortion  that a binary BH merger  in a nearby galaxy induces in space-time is minuscule when it reaches Earth. So minuscule that LIGO need to measure tiny shifts in the relative position between two mirrors which are several orders of magnituude smaller than the size of the smallest atom. This is an incredible achievement.  LIGO’s first detections of GW have brought a few surprises though. And they started with a bang!

The first event detected by LIGO in 2015 was interpreted as a heavier than expected binary BH merging in a  closer than expected galaxy. Similar events have been observed since raising several questions. Are these events more common than previously thought? Why have not we see them farther away if they are stringer than expected?  The mass of the individual black holes forming the binary BH were inferred to be approximately 30 solar masses each. Note that I say inferred because these masses could not be measured directly. What LIGO can measure with relative high precision is what is known as the observed chirp mass.  The intrinsic chirp mass is some combination of the two masses of the binary black hole. If both masses are similar, the chirp mass is similar to those masses as well. If the two masses of the binary BH are very different, the chirp mass will be a value in between these two masses (but closer to the mass of the lightest component). The observed chirp mass is related with the intrinsic chirp mass by the factor (1+z) where z is the redshift of the binary BH. The redshift is a measure of the distance so more distant objects have a larger redshift (our redshift is zero).  In other words, what LIGO can measure with good precision is Mo=Mc*(1+z) where Mc is the intrinsic chirp mass. Mo determines the frequency at which the GW is oscillating, a number that LIGO can estimate quite well. For that very first event, LIGO found that Mo had to be approximately 30 solar masses and that the distance was relatively small, that is z was close to zero. Hence, the intrinsic chirp mass (and mass of the individual BH before they merged)  had to be also close to 30 solar masses. This came as a surprise since many predictions made years earlier anticipated that such high values for Mo should be very rare. In fact, what was expected was to find values for Mo between 7 and 15 solar masses. This was in part motivated by observations of X-ray binary stars in our Galaxy, for which  it is possible to estimate the mass of the BH. An X-ray binary is a pair of closely orbiting  objects where one is a star and the other one is either a neutron star or BH. In this article we consider only the BH case. Roughly speaking, by measuring the amount of light emitting by the gas (from the star) spiraling towards the BH one can measure the mass of the BH.  In our Galaxy, the mass of about a dozen BH has been measured using this technique. The results show that the BH masses are between ~7 and ~14 solar masses. So far, no BH with a mass higher than 20 solar masses has been found in our Galaxy raising another, even more fundamental, question. Is our Galaxy special or is there something else we are missing regarding the BH masses and distances inferred by LIGO?

This is the question we address in our latest work. Owing to the degeneracy with the redshift described above, would it be possible that the intrinsic chirp mass was smaller if the redshift was higher? If the redshift is, let’s say z=1, instead of z~0 then, the intrinsic chirp mass could be a factor of two times smaller than the value inferred by LIGO (while keeping the observed chirp mass constant) bringing it into agreement with the BH masses observed in our Galaxy. There is one caveat though.  If the GW was originated in a galaxy far away at redshift z=1, instead of in a galaxy nearby (z~0), the intensity of the GW would have been much smaller than what LIGO observed. The intensity of the GW  is (to first order) the quantity that is used by LIGO to determine the distance. The observed intensity determined then that the inferred distance had to be relatively small. A mere few hundred Mpc instead of several thousand Mpc which would be the distance for a galaxy at redshftz~1 so one would conclude that the GW originated in a nearby galaxy and consequently, the intrinsic chirp mass had to be high. But this is the funny thing. Nature has interesting ways of playing with us. One of these ways is gravitational lensing thanks to which, an object that is far away may appear to us as if it were much closer (that is, it can amplify the intensity mentioned above). Note that I used the expression infer again when referring to the distance estimation by LIGO. This estimation is made under the assumption that gravitational lensing is not intervening. This is normally a good assumption since, after all, only a very small fraction of distant objects get (significantly) affected  by gravitational lensing. To be more precise, 1 in approximately 1000 or 10000 objects at redshifts larger than z=1 are substantially magnified by the gravitational lensing effect. Hence, is it still possible that a significant fraction of the LIGO events are distant lower mass events that are being magnified by gravitational lensing?  In our work we find that lensing can just make the trick. At large distances, the volume of the universe that is reaching us now (and by this I mean the volume where the light or GW we see now originated) is much  larger  than the corresponding volume at much smaller distances. To visualize this, imagine the volume of a shell of radius R. This volume goes like the square of the radius. So a large shell with a radius 10 times larger than a smaller shell will have 100 times the volume (if they both have the same thickness).  By precise calculations of the gravitational lensing effect over distant gravitational waves we prove that the massive and nearby events found by LIGO can in fact be interpreted as normal but more distant events with masses comparable to the ones found in our Galaxy. This solves the puzzle mentioned at the beginning of this article. Is our Galaxy special? And if it is not, where are the masses that LIGO claims is finding in nearby galaxies? The answer is that those masses would be the same in our Galaxy and in other galaxies. What is wrong is the interpretation of the observation since the amplification due to lensing has been ignored (this story is very similar to the puzzling first bright galaxies detected by Herschel that turned out to be all gravitationally lensed distant galaxies) .

So why has not anybody realized this earlier? That is a good question and the answer is not because people have not thought about this before. For our model to work, there is one little thing that sets our study apart from other similar attempts. As we mentioned earlier, at z~1, only one in a few thousand events could be magnified substantially by gravitational lensing. On the other hand, by observing more distant objects one is observing a larger volume, so one is observing more events. The gain in volume with respect to nearby distances is in the range of two orders of magnitude (more precisely about 1.5 orders of magnitude between z=0.1 and z=1 for a shell of thickness dz=0.1). This gain in volume is not enough to compensate the small probability of lensing at z~1 (1/1000 or less). A significant rate of lensed  events (enough to explain the rate of observed events)  can be obtained ONLY IF (and this is the little thing)  one increases the rate of intrinsic mergers at z=1 with respect to the rate at z=0. Such evolution in the intrinsic rate is expected and has been considered in the past. Our study shows that in order for the lensing mechanism to work and be able to explain the LIGO observations (with the troubling masses), the rate at z=1 needs to be more extreme than previously considered. This is not necessarily a problem since we simply don’t know what this rate is and also there are models that predict such rapid evolution in the intrinsic rate of events between z=1 and z=0 but, surprisingly, this type of strong evolution models were not considered in the past so the role played by lensing  was not recognized.

So then. Are we right? Are we wrong? Time will tell. After all, only one (if at all) of the many interpretations proposed to explain the LIGO massive events will be the correct one. An important aspect of any model is that it needs to be testable and this one is. If lensing is the culprit, at high magnifications one would expect a pair of images with similar magnifications and with a small time delay between them (hours to days depending on the lens mass, lens distance and relative source-lens-observer position).  LIGO detections don’t come in pairs (at least no such detections have been reported yet). If the time delay is several hours or days, it is possible that one of the two lensed events falls below the detection threshold of LIGO since the visibility (determined in part by the geometric factor in LIGO, a technicality whose explanation is beyond the scope of this article) may have changed substantially.  For simplicity, we can say that an event that is directly overhead the detector results in a significantly stronger signal-to-noise ratio than the same event near the horizon. Since Earth rotates once every 24 hours, a position in the sky (like the Sun for instance) can move from the zenith to the horizon in six hours. Hence, two identical GW originating in the same spot in the sky may have significantly different signal-to-noise if they are separated by approximately six hours. There is however a limit for how many times you may get the unlucky configuration that permits to hide one of the two images. Eventually two events should be observed that have virtually the same observed chirp mass and a distance estimate that is consistent with the uncertainties introduced by the geometric factor. The ratio of signal-to-noise between the two events should be compatible with the angle rotated by Earth during the time separation between the two events. Finally, the inferred location in the sky (derived from the time difference between detections in different observatories) should be also consistent with being the same for both events. Data mining of the LIGO data may unveil some of these missing events in the near future and confirm the lensing nature of the massive LIGO events.

Link to the publication

You can download the paper with our study in this link







Solving a long standing mystery

Planck helps solve a long standing mystery

Virgo cluster (marked with a big circle) is near the centre of the image. The signal detected by Planck extends well beyond the limits of the cluster probing part of the missing baryons.

One of the puzzles of modern astronomy is what is known as the missing baryon problem. Baryons are the ordinary matter we are familiar with. You are made of baryons as it is everything you touch, eat and see. The best known form of baryons are electrons and protons. Together with neutrons (another form of baryons) they form atoms and atoms form molecules and molecules form … well, everything else. Detailed observations of the distant Universe tell us how many baryons are out there and the amount we can see agrees very well with what is expected from the standard model that describes the Universe so there is nothing surprising there. The story changes when we look at the Universe but at distances much closer to us. In theory, we should see the same amount or proportion of baryons that we see in the distant Universe right here, in our neighborhood but they are no where to be found, so where are they?

Baryons follow a similar law than energy, they don’t get created nor destroyed (for the most part), they transform  (with the transformation between a neutron and an electron plus a proton or viceversa being a classical example). If there were baryons in the early universe, pretty much the same number of baryons should exist today. Instead, observations of the local Universe reveal a significant deficit of baryons when compared with the expectations and the observed number of varions in the most distant Universe.   It is commonly believed that most of these missing baryons are in the form of a plasma which emits very small amounts of light (mostly at high energies like UV or X-rays) which has not been detected so far. Howevere, the same plasma produces also a distortion in the light that originated soon after the Big Bang (more rpecisely, 300000 years after the Big Bang). This light, known as the Cosmic Microwave Background, or CMB,  has been travelling through the Universe since the time it was first produced and permeates the entire Universe. When the CMB light crosses a region filled with plasma, it gains a small amount of energy. This small gain of energy can be measured with current telescopes like the Planck satellite through an effcet known as the Sunyaev-Zel’dovich, or SZ ,effect. The SZ effect has been studied with Planck in dense and hot plasma regions, usually found at the centre of galaxy clusters. In a recent work, we have focused our attention to one particular cluster, the Virgo galaxy cluster. This cluster is special because it is the closest cluster to us. In fact, it is so close that  our galaxy is falling towards the centre of this cluster due to its ginat gravitational attraction. The distance from our galaxy to the centre of Virgo is only about six times  the distance from our galaxy to our closest sister galaxy, the Andromeda galaxy. The apparent size of Virgo in the sky is about 15 times larger than the apparent size of the full moon. This large size, allowed us to do a detailed statistical analysis that takes advantage of the large size of Virgo and maximizes the small distortion that the missing baryons around Virgo produce over the CMB light.  Our findings (summarized in the figure accompanying this post) reveal vast amounts of plasma beyond the previously established limits of the Virgo cluster. The signal around Virgo observed by Planck coincides with what was the expected signal emerging from the missing baryons around galaxy clusters confirming that the missing baryons are probably forming diffuse clouds of plasma around the biggest structures in the Universe, like galaxy clusters. Although the missing baryons found by Planck don’t account for all the missing baryons, it does reduce the amount of baryons that are still evading a firm detection. Future analyses based on Planck and ground-based experiments will continue  the hunt for the few remaining missing baryons …


The paper with all the details and results can be found in the following link :  http://arxiv.org/abs/1511.05156




Hubble Frontier Fields Program extended (and time travel)

SN_predictionhe Hubble Frontier Fields (HFF) program has been recently extended to include two additional clusters to this spectacular data set. This is great news for science. To date, the HFF has provided the best data set to study the distribution of dark matter in galaxy clusters (the same data set is used for other exciting projects). Currently, the HFF is covering two clusters (from the original set of 4). One of them is MACSJ1149.5+2223. This cluster is interesting for several reasons. One of them is the fact that a supernova at z=1.491  was observed by Hubble in one of the observing campaings. This supernova is seen 4 times, in a configuration known as Einstein cross. The multiple images observed by Hubble are distorted versions of the original supernova that are multiply lensed 4 times (gravitational lensing). One interesting feature of gravitational lensing is that since the paths of photons are distorted by the gravitational potential, and so is their time of arrival to our telescopes on earth. Because of this time difference, multiply lensed images of the same background object are seeing in different epochs. Is like seeing your kids simultaneously when they had different ages, … weird. The supernova observed by Hubble is observed (4 times) in one of the arms of its host galaxy in just one of the counterimages but not in the other two counterimages. This means that that supernova will be observed in the future in the other two counterimages or it has already happened in those two counterimages. Since the lifetime of a supernova is short (days or weeks) and there is no way to predict when a star will go supernova it is nearly imposible to observe a star going supernova before it happens.  The multiply lensed images  of the supernova in this cluster, could in theory, allow us to predcit when a supernova is going to be observed and study the supernova explosion from the very beginning. Using accurate models of the gravitational potential we where able to predict the time difference between the different counterimages and predict when the supernova will be observed next in the different counterimages or when it was happening in those images. The figure shows the predicted time delays for the supernova observed in MACS1149. This supernova is observed now four times. Our model predicts that we are too late to observe one of the counterimages that occurred about 9 years ago but a new chance to see this supernova will take place again around November 1st 2015.  This will be the first time we can point a telescope to a position and wait for the SN to happen (again). Talk about time travel !

Original paper: http://arxiv.org/abs/1504.05953

Published version: http://mnras.oxfordjournals.org/content/456/1/356


On December 12th 2015, news broke about the reappearance of SN Refsdal at the exact predicted position posted in this article. The date of the explosion is uncertain by one month but it must have happened between November 15th and December 10th which are the dates when Hubble was observing at this location. On the November 15th observation there was no sign of the explosion but in the December 10th observation the SN had already shown up at the predicted position. The date of the original prediction (November 1st) is based on a value of the Hubble constant that is a bit out of date (h=0.7). Adopting a more recent estimate (h=0.67) and re-scaling the time delay, the best prediction for the reappearance shifts from November 1st 2015 to  November 17th, right in the window of time where we know the explosion had happened.




21st century is here

INdiaMarsMissionWhen was the first time you noticed you where in the 21st century? For some of you it may have been when you owned your first smartphone, or your first electric car, or your fancy 3D TV or 3D printer, or maybe when you tried the google glass? Those are cool little things that may impress you for a while but hardly they’ll make it into the history books. Now really, when was the first time you had a sense that things had already changed? For me that day was this week when I saw this picture. It does not just tell a story, it shows the change that will dominate the 21st century.  The Indian Space Research Organisation succesfully put in orbit a small satellite around Mars. The picture shows a group of female scientist/engineers celebrating this tremendous achievement.  This picture illustrates the game-changing rules of the 21st century. First, the definite raise of Asia as a superpower in the world. Space missions are normally used by governments as powerful messages to the world. Very few things have the power to bring the attention of the world (in a good way) as a successful space mission. One of the reassons why governments choose space missions to demonstrate their power (economic, technological, militar and even political) is that you can not cheat in space. You either have the will and technology to be succesfull  or you don’t. India, together with its Asian neaighbours,  is destined to play a leading and fundamental role in the 21st century. But even more imporant, is the second conclusion that we can get form the picture above. What comes to your mind when you think of a rocket scientist? Well, think again. Women also, are destined to play a fundamental and leading role in the 21st century. The 20th century was full of promisses for women, some of them only partially fulfilled. Now reality is here. If one thing will define the 21st century it won’t be the defnite raise of Asia, but the definite raise of women.

MOND and Dragon Kick galaxies.

DragonKick_MONDMOND (MOdified Newtonian Dynamics) models are popular among some scientist because they avoid one of the biggest problems in cosmology, dark matter. This misterious substance remains undetected although there is evidence from several observations that it must exist and in vast quantities. MOND models are able to explain some of these observations by modifying the laws of gravity. In particular, at cosmic distances MOND models propose that the gravitational acceleration does not decay as the inverse of the distance squared but at a smaller rate. This slower decay of the gravitational acceleration would effectively describe some of the observations without the need to invoke the existence of dark matter but it also has its own problems, like fine tunning of the parameters in the models. Together with some collaborators, we recently studied a particular galaxy behind the cluster MACS0416 that is gravitationally lensed (or bended) by another galaxy in the same cluster. We named this galaxy the Dragon Kick galaxy because it rejects the MOND hypothesis and confirms the pressence of  a halo of dark matter around the lens galaxy. The Dragon Kick galaxy is shown above as a blue arc that is  super-impossed on the legs of our would be Bruce Lee. Our results will be made public next week but basically we find that the lens galaxy (shown above as a yellowish edge-on galaxy  emerging from the private region of our Bruce Lee) requires a halo around it that aligns perpendicularly with the lens galaxy in order to explain the shape of the lensed blue arc (the Dragon Kick galaxy) . The mass of this invisible halo (the dark matter) is larger than the mass of the lens galaxy and in agreement with what is expected from the standard model that assumes the existence of vast amounts of dark matter in the universe. More Dragon Kick galaxies are expected to be studied soon that could help in providing new clues about the nature of dark matter.

 You can see the original paper here: http://arxiv.org/abs/1409.1578

Science in the 21st century. What can we learn from the BICEP2 experience?

BICEP2It was a big deal in March 2014 when the BICEP2 collaboration announced the detection of primordial gravitational waves, the echo of the Big Bang. A potentially Nobel-winning discovery, later on the same authors, following pressure from the community and evidence of systematic problems in their analysis, admitted that the detection could not be claimed yet and that more data is needed to settle the issue. The European-led Planck mission is expected to clarify the situation later this year. However, whether Planck is able to confirm or reject the hypothesis that gravitational waves where detected, what we know now is that the announcement of the alleged discovery  was premature. This rush for being the first is becoming a growing problem in science that can backfire the scientific community. If the BICEP2 result turns out to be the result of an analysis based on incomplete crucial information, the same publicity the BICEP2 collaboration got when they announced their results may turn back but not just on them but on the entire community.  Public funding is the base for science and will continue being so in the future. The impression the general public gets on how their funds are being spend by the scientific community may determine the amount of funding available for future projects. If scientist  act irresponsibly and rush their results after the sought recognition and the results turn out to be wrong, this will  affect the entire community. It is difficult to control these situations as scientist often suffer of a delusional disease where they believe their results because  they want to believe them. Consequently, in most cases they are acting in good faith when announcing their results but they should always leave the door open to the possibility that something went wrong and honestly point to the uncertainties that may be affecting their conclusions. The BICEP2 collaboration gave the impression for a while that this door was shot and locked but cracks appeared in the door soon after their announcement as a consequence of the pressure form the rest of the scientific community. Planck will soon go through that door and leave it close or wide open …