Clever ways of observing invisible stuff
Imagine a new form of energy, or perhaps a new constant of Nature, filling the whole Universe and yet so mysterious that it cannot be studied in any laboratory experiment. Imagine it having the most puzzling properties, such as a negative pressure which is accelerating the expansion of the Universe. Imagine something so strange that Einstein himself, who first saw the possibility of its existence, later reportedly called it ?my greatest blunder?. This is one of the greatest unanswered questions of contemporary physics: the nature of dark energy.
About a decade ago, cosmologists where stunned by a study looking at immensely powerful explosions of distant stars at the end of their life, called supernovae. A supernova explosion is so bright that for a brief time it can outshine the entire galaxy to which it belongs, and therefore it can be observed billions of light years away. To cosmologists, supernovae are very interesting objects because they act as beacons of known luminosity, which signpost the expansion history of our Universe. By measuring the dimming of their light, we can infer how much the Universe expanded since the supernova went off. The amazing discovery was that in the last couple of billions years the expansion of the Universe did not slow down, as one would expect if it only contained matter and ordinary radiation (i.e., light). Rather, the expansion had been accelerating, under the influence of an unknown repulsive effect. The first hint for the existence of dark energy had been found.
Since then, cosmologists have worked hard to devise other inventive ways to pin down dark energy. If supernova explosions serve as “standard candles”, then perhaps one could use “standard rulers”, as well. In theory, the recipe is simple: find a “cosmic yardstick”, i.e. some astrophysical object of known length, and observe it at different epochs in the cosmic history. Its apparent size will then tell us about how much the Universe has expanded in between, and in particular if dark energy has given the Universe an extra stretch. In practice, such a cosmic yardstick must be of immense length, but fortunately nature has provided us exactly with such a tool, in the form of so called “acoustic oscillations”.
This is the name cosmologists give to sound waves in the primordial Universe, when the temperature was high enough that only a hot plasma of elementary particles could exist. When the Universe cooled down sufficiently, the ripples of the sound waves got frozen, and it is precisely around the peaks that galactic structures tended to form preferentially. Therefore, the separation between galaxies today should show a preferred distance of about 500 million light years: a perfect cosmic yardstick! The reason why we know precisely how long the cosmic yardstick is today is that we have measured it in the very distant past, thanks to observations of the cosmic microwave background, the relic radiation of the Big Bang. In fact, the distance between a peak and a trough of the primordial sound wave is reflected in the separation between hot and cold spots in the microwave background, and this can be used to calibrate the yardstick. The existence of acoustic oscillations has now been proved, and in the future observations of millions of nearby and distant galaxies will reconstruct the length of the yardstick deep in the cosmic past.
What about the possibility of using our knowledge of the growth of galactic structures to track down dark energy? Here a major hindrance is that most of the mass in the Universe is in the form of dark matter (about 5 times more dark then luminous matter). But this can actually be used to our favour, by exploiting the fact that the gravitational attraction of massive objects – both luminous and dark – deflects the path of light passing nearby. Thus the image of a distant galaxy as observed on Earth will be distorted, because the light has been slightly deviated from its path by the gravitational effect of the dark and luminous matter between the galaxy and us. This is what in cosmological jargon is called “gravitational lensing”. By analyzing a great number of images of galaxies, we can reconstruct the distribution of matter at different epochs in the cosmic history. The theory of gravitational collapse can then be used to link these observations to dark energy. For instance, the presence of dark energy slows down the growth of structures, and this will be apparent from future gravitational lensing observations.
Combination of evidence from all those different techniques points strongly towards the existence of dark energy, perhaps in the form of Einstein’s cosmological constant. Faced with one of the deepest mysteries of modern physics, cosmologists are now planning a new generation of bigger, faster, and more accurate surveys, which in the next decade will enable us to shed new light on the dark side of the Universe.
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[Response] Although I am not aware of studies of conceivable ways of harnessing dark energy as an energy source, it is clear that the existence of dark energy itself imposes a fundamental limit to the efficiency of any energy consuming process in the Universe. Any thermodynamical process requires a "heat sink" to work - ie the possibility of discharging the heat produced in the process onto some cooler object, eg the environment. This is valid for the fridge in your kitchen, for the engine of your car, and for the cells of your body. The existence of dark energy sets a fundamental limit to how cool the Universe will ever become, thus giving a limit to the ultimate heat sink of all, ie outer space. Thus given a certain amount of energy that we want to convert into thermodynamical processes, the existence of dark energy sets a limit to how efficiently we can do so. So in this respect dark energy is actually working against us!
Anthony Ellis Birmingham UK
[Response] It might be that dark matter and dark energy are an expression of an underlying, common phenomenon. There are models that try to explain both by introducing a new "substance" that would behave like dark matter in the past, but would then appear to be dark energy today. However, this would not account for other pieces of evidence of the gravitational effect of dark matter, such as flat rotation curves in galaxies and gravitational bending of light. Another hint of the fact that dark matter and dark energy might have something to do with each other is the so-called "coincidence problem" - namely the surprising fact that the density of dark matter and darke energy are about the same (within a factor of 3) at this point of the cosmic history. This will not be the case in the future (when dark energy will presumably dominate), nor was in the past (when dark energy was negligible and dark matter predominant). The reason why (if any) we are witnessing this crossing over among the two epochs at about this cosmic time remains an unsolved problem.
[Response] There is indeed no edge to the Universe, in pretty much the same way as an ant on the surface of an expanding balloon would not be able to perceive any edge to its surface. If we stick with the analogy of the expanding balloon, and if we imagine sticking pennies on the surface representing galaxies, then we see that as we inflate the balloon all of the pennies (ie, the galaxies) keep moving further apart from each other. If we take any given galaxy, we will still observe all of the other galaxies moving away from it. This of course does not mean that the particular galaxy we have selected is "at the centre" of the expansion: all of the Universe is expanding in the same manner. Therefore we can determine that the Universe is indeed expanding by observing the recession velocity of galaxies around us. We do observe that they are in fact "moving away" from us - this is called "cosmological redshift". What dark energy does is to accelerate the rate of this expansion, that ought normally to slow down due to the mutual gravitational attraction of the matter in the Unvierse. One way of determining the acceleration of the expansion is to use "standard candles" such as supernovae type Ia explosions.
[Response] If I interpret correctly your questions - yes, it would be theretically possible if the Universe was closed, ie the 3D analogous of the surface of a sphere (do not try to picture this in your head, you'll find it is thoroughly impossibe!). If the Universe was closed, then at some point in the future it would collapse under its own weight, start contracting and finally undergo a Big Crunch, which is the exact reverse of the Big Bang. But for the Universe to be closed, it needs to contain enough matter (both ordinary and dark) so that its gravitational attraction will make it contract in the future. This does not seem to be the case for our Universe, which only contains about 30% of the matter that one would need to achieve a Big Crunch in the future. The rest is made of of dark energy, which actually is making the expansion accelerate. Therefore we can predict that our Universe will continue expanding forever and will not end in a Big Crunch.
[Response] You are right - there are indeed galaxies heading towards us, and as you correctly say, the mutual gravitational attraction of galaxies and clusters makes them move around in a complicated way, falling towards each other, fragmenting, merging etc. The velocities associated with this phenomena are called in jargon "peculiar velocities". But the total velocity of galaxies in an expanding Universe is given by the sum of such peculiar velocities and the cosmological recession I was describing. Now the distinctive feature of the Hubble expansion of the Universe is that the recession velocity is proportional to the distance. That means that galaxies further away from us are receding faster due to the cosmological expansion (and yes, in case you are wondering, galaxies that are distant enough will recede faster than the speed of light - but this could be the topic of another post). So for distant galaxies the cosmological recession is far greater than any peculiar velocity they have. For nearby galaxies (such eg the Andromeda galaxy, some 2.6 million light-yrs from us) their peculiar velocities are dominant, and you will find that some of them are indeed falling towards us.
[Response] Yes, dark matter does attract other massive object gravitationally and indeed its effect is seen in the way stars spin around in galaxies and the way galaxies spin around each other - ie they are all held together by the extra attraction coming from dark matter. Perhaps the confusion arises because you are mixing up two different things, ie dark matter and dark energy. They have two completely different effects, dark matter being attractive (gravitationally), while dark energy being repulsive.
[Response] In principle you are right - there is a moment in the history of the Universe where dark energy and dark matter exactly balance out, when the Universe transitions between decelerated expansion (under the influence of dark matter) and accelerated expansion (when dark energy takes over). However, the thing is, as the Universe expands the density of dark matter decreases, as the dark matter particles are diluted in larger and large volume of space. On the contrary, the density of dark energy remains constant with time, if dark energy is in the form of a cosmological constant (you might understand this by thinking of dark energy as energy associated with empty space - the more empty space you have, the more dark energy there is around). So eventually dark energy always becomes to dominate the Universe, which is what is happening right now (dark energy is about 3 times more abundant than dark matter). Therefore the Universe does not stop expandind, but it expands at a faster and faster rate.