the biggest news in cosmology in# recent years is that the mysterious universe accelerating entity that# we call dark energy may be fading away the evidence for this is now strong# enough that enormous effort is going into confirming the result so what's it going# to take and when are we going to know in the '90s we thought cosmology was solved the# universe is expanding and slowing down under the influence of gravity in an effort to nail down# this result astronomers tracked the expansion history of the universe by watching the flashes# of distant supernova they found that expansion wasn't slowing down at all it was speeding up# we don't know what mysterious influence causes this acceleration but we've named it dark energy# the most mainstream interpretation is that the vacuum of space has a native energy to it quantum# mechanics predicts this to be the case even if it can't predict the quantity this idea also gives# us the simplest mathematical description of dark energy the addition of a single number the# cosmological constant lambda to the Einstein equations and we saw in another video that# this result is a constant energy density and a resulting exponential expansion this simplest# version of dark energy is part of the lambda CDM cosmological model and historically it's done# a pretty good job of describing the expansion history and the growth of structure through# cosmic time with lambda CDM we thought that we were back to cosmology being mostly solved# time to tie up the loose ends once again by refining the measurement of the expansion history# to verify the dark energy is indeed constant many thought that the dark energy science instrument# would do just that confirm lambda CDM it didn't desi seems to be showing the dark energy isn't# constant at all rather it may be decreasing in strength which puts in doubt the whole vacuum# energy thing we first reported this result last year after Desi's first data release when the# hint of a changing dark energy was just that a hint desi's second data release has now promoted# this tenuous result to a confidence level just shy of a formal detection that was a few months# ago now and so this is not a news piece about the new result instead this episode is really about# what comes next what measurements can and will we make next to confirm that dark energy is changing# to better measure how it's changing and what this can tell us about what dark energy really is that# said a quick overview of DESIE is in order the dark energy science instrument is an instrument# it's the survey conducted by that instrument and it's the nearly 500 person collaboration behind# it all the instrument is on the male telescope at Kit Peak in Arizona it robotically places 5,000# optical fibers to simultaneously capture the light of as many galaxies that light has traveled to# us for billions of years in some cases through an expanding universe and the signature of that# expansion is imprinted in the stretching or red shift of each galaxy's light by splitting the# light of each galaxy with a spectrograph the Desi team can measure that red shift which encodes# both the distance to each galaxy and the expansion history during that light's journey these two# properties can't be disentangled without more information and so Desi does something else# incredibly cool remember we want to measure the expansion history perhaps the simplest way# to think about it is that we want to measure how big the universe was at different times in# the past light takes time to reach us so light from distant galaxies carries a snapshot from# an earlier time next we just need some sort of structure that has expanded with the universe# so we can measure the size of the universe at different times in the case of Desi that something# is the baron acoustic oscillations Baos which we discussed in previous episodes these are gigantic# circular patterns in the distribution of galaxies on the sky they are the remnants of sound waves# in the hot dense plasma that filled the early universe before the first galaxies formed these# density waves froze at a certain size when the first atoms formed leaving a ripple pattern in# the gas and dark matter that filled the universe the galaxies that would later form from that# matter still reflect that patterning but the ripples have grown with the universe they now# give us a powerful standard ruler to track that expansion so simplistically we have red shift that# encodes the subsequent expansion history from a given time in the past and then we have a measure# of the scale of the universe at those times this doesn't quite give us an unambiguous expansion# history by itself but the combination does allow desi to test the consistency of different models# of that history different cosmological models in the case of Desi the cosmological models tested# include lambda CDM with its constant dark energy as well as a particular model for changing dark# energy in that model dark energy changes over time according to an equation given in the form of# a varying equation of state parameter something we talked about in the previous video by itself Desi# finds that the model with changing dark energy has only a marginal improvement over lambda CDM but# things get much more interesting when the desi data is combined with other studies the first# is the constraints from the cosmic microwave background radiation which provides the starting# point the initial conditions from which cosmic expansion and structure formation measured by desi# will evolve the other is an independent measure of distance desi itself only has the galaxy red shift# as a distance indicator which makes it difficult to pin down the absolute scale of these BAO rings# so they add an independent measure the same type that was first used to discover dark energy type# 1A supernova these exploding white dwarf stars are called standard candles because they explode with# predictable brightnesses that means we can figure out how far away they are based on how faint they# appear due to that distance one way to think about it is that the supernova give distances from# us to the galaxies while the barrier acoustic oscillations give distances between those galaxies# standard candles plus standard rulers combined with the initial conditions from the CMBB these# measurements give a much tighter constraint on their cosmological parameters desi claims that# the combination favors a cosmology in which dark energy is actually weakening and that this differs# from the cosmological constant model by up to 4.2 sigma for the supernova in Desi's own survey but a# bit less than 4.2 for other supernova surveys this doesn't quite reach the five sigma level required# to claim a discovery but it's tantalizingly close so did Desi probably discover that dark energy# isn't constant not really besides not quite hitting five sigma they also test this one version# of varying dark energy which you could argue is a little ad hoc when you're fitting a curve to a# bunch of points adding extra degrees of freedom always improves the apparent fit that could be# what's happening here still it's intriguing enough to warrant deeper exploration what we really want# is a precision measurement of the way expansion and dark energy have changed over cosmic time# because that would allow us to a be sure that the change is really happening and b distinguish# between different models of dark energy which in turn could tell us what dark energy really is now# we've talked about those options before so I won't go into it now i want to focus on what's needed# and what's being done to improve this measurement how do we improve the desi result well the answer# is a little prosaic I'm afraid we just need to do everything better let me remind you of the# three things we talked about now this is an oversimplification but it serves to organize# the ideas we need to one measure the starting state of the universe two to measure the size# of the universe at different times and three to measure distances to those different times we'll# dispense with number one quickly measuring the starting state means measuring the initial density# distributions and various energetic components of the universe from the cosmic microwave background# there are plans to do this better than the plank satellite already has but honestly our current# CMB map is the most precise part of this whole calculation so it's not the limiting factor so# straight on to number two how can we improve our measurement of the size of the universe well# Barryon acoustic oscillations really are the gold standard and so just doing desi better is# probably the way to go that means we need bigger better galaxy red shift surveys which beat down# the uncertainty in various ways including the sheer number of galaxies we also want to push# the measurement back further in time which we can do with more sensitive instruments on bigger# telescopes or just staring at the sky for longer to collect photons from fainter galaxies and# three we also want to improve our measurement of distances to those galaxies the most obvious thing# is to improve our supernova measurements with more type 1A but also by refining our calibration of# these things as standard candles this is one of the most contentious points in the whole effort# because the supernova distances piggyback off a chain of distance measurements and so errors can# have crept in unnoticed in various places major efforts are underway to weed out potential issues# it's also important to find independent ways to get those distances one of the most exciting and a# personal favorite of mine is to use gravitational lensing gravity bends space and so bends the# path of light very strong gravitational fields like those of entire galaxies bend space so much# that the light from a distant source can travel to us via multiple separate paths for example# distant quazars feeding super massive black holes sometimes appear duplicated or even quadrupled# around a galaxy that happens to lie directly between us and it now when that quazar fluctuates# in brightness all of these lensed images also fluctuate because they're all the same quazar but# the pattern of fluctuations has an offset in time that reflects the difference in the lengths of# these paths measuring this time delay actually gives us a measure of the cosmic distances# that's completely independent of the supernova measurements astronomers have used a handful of# lensed quazars to measure the expansion rate of the whole universe but by doing this for many more# such objects we can also get the expansion history now I was being a little simplistic when I# separated the ideas of measuring the size of the universe at different times to measuring# the distance to those times really any given cosmological measurement does some of both# for example we track the size of the universe just by looking at how far apart galaxies are in# general not just the size of the BAO rings so this size information is also there for example in the# supernova cataloges even without the BAO analysis and the expansion history is coded in the red# shifts so we can calculate expansion rates even from just say the lensing time delays but really# combining these data sets is the key what we're trying to do is fit a cosmological model with many# interacting or covariant parameters so we need measurements that will tease apart and constrain# all parameters and on that note there is one parameter that I haven't mentioned and that's the# clustering of galaxies over cosmic time galaxies are thrown apart by cosmic expansion but they also# fall together due to their mutual gravitational attraction the amount of this clustering tells# you stuff about the amount of dark matter in the universe and the size of the initial density# fluctuations both important parameters in your cosmological model so measuring this clustering# is a big help in constraining that model galaxy red shift surveys like desi can do this directly# to some extent based on the clustering of the galaxies that they can see but there are also# more cunning approaches that can take account of all the mass seen or unseen for example through# weak gravitational lensing the lensing I already talked about where you see multiple images of one# object is called strong lensing in weak lensing the gravitational field of galaxies and clusters# slightly warp the shapes of background objects without actually producing multiple images in the# case of a weakly lensed galaxy it'll be slightly stretched in one direction you can't identify# the effect with a single galaxy because you don't know what shape that galaxy was originally but# imagine many galaxies in a particular direction that normally have random orientations relative# to each other as their light travels to us it passes around let's say a particularly dense# patch of the universe the orientations of all those galaxies will be skewed at right angles# to that patch and become correlated with each other by looking at the correlations of galaxy# shapes across the entire sky you can get a crude density map of the universe which in turn allows# us to constrain galaxy clustering the desi result that made all this news is from their second data# release 3 years into the 5-year survey and with 30 out of the planned 50 million galaxies and# quazars actually if the deviation from constant dark energy is real it seems likely that this# extra signal will get them over the five sigma level but Desi is not the only game in town nor# the only thing needed to reduce that uncertainty we should not confuse Desi with DE for example# that's the dark energy survey des observes a factor of 10 more galaxies than Desi but where# Desi takes spectra of known galaxies Des takes images at different wavelengths and actually# discovers galaxies that makes DE an imaging or phototric survey where DESIE is a spectroscopic# survey de measures phototric red shifts which are less accurate than DESI's spectroscopic# red shifts but take much less time to observe des also doesn't see as far as DESIE so isn't# sensitive to the expansion history instead DES is good at measuring the latetime expansion rate# as well as the growth of structure via clustering and weak lensing measurements it's also important# for nailing down them pesky supernova really DESI is complimentary to dese giving the late time# constraints just as the CMBB gives the initial conditions but perhaps the greatest improvement on# these results will be from very new and upcoming surveys we have the European Space Ay's Uklid# satellite which is studying over a billion galaxies with both imaging and spectroscopy and# his most important cosmological contribution is by measuring galaxy clustering and weak lensing this# should allow us to beat down the uncertainty in the equation of state parameters to the 1% level# which moves us toward a precision measurement of dark energy uklid is in the first of its six-year# mission and the first cosmological results are expected next year and the last and perhaps for# me most exciting endeavor is the legacy survey of space and time LSST on the Reuben Observatory this# is a phototric survey that does something very new it'll map much of the southern sky many many times# for 10 years besides complmenting or improving on desi deaths and uklid lsst will add the dimension# of time to the analysis perhaps the most important benefits to cosmology are the measuring of# many hundreds of thousands of type 1A supernova providing an incredible late time anchor for the# other surveys lsst will also enable measurement of those time delays in the gravitational# lens quazars which if you remember give us an expansion history independent of supernova# but LSST will only do this after discovering something like 20 times more lensed quazars than# we currently know of so this method should really come into its own with LSST we are entering the# era of precision cosmology it's really a golden era for the field and in this next decade you# can expect the cosmic expansion history and the general cosmological model to become known# to a precision that seemed impossible not long ago with that precision comes the ability to ask# what kind of physics can produce such a universe what kind of dark energy dark matter starting# conditions and what does that mean for how the universe will end we will know it all just as# soon as we've finished surveying all of spaceime