In this week ’s “ Ask a Physicist , ” I get justificative about dark energy . It dominates the universe ; it ’s all nonsensical ; and it ’s apparently absolutely necessary .
I ’ve been putting off verbalise about sour energy for a while . I was afraid that you ’d judge me , and all cosmologist , as being charlatans . But I ca n’t put it off any longer , so for this week ’s “ demand a Physicist ” our ( metaphoric ) pity will be revealed by Donovan , whoasks :
I ’ve never seen someone endeavor to explain sorry energy as anything other than a feature of our population . rather of explain dismal energy as a mysterious force inside of our universe that is pushing out and inflating us , has anyone ever tried to excuse it by call for some form of “ vacuum ” outside of the population that is “ pulling ” us out ?
You see ? This is why I ’m so defensive . The idea of dark energy is so ridiculous that almost every doubtfulness is based on trying to make it go away . And trust me , I share your concerns . I do n’t desire to believe in dark vigour , but I have no option .
When I was a scholarly person in the mid 1990 ’s , cosmologist consider we had it all figured out . dark-skinned subject had been known ( or at least suspected ) since the 1920 ’s , and most sizable physicist only assumed that the universe was made up of a combination of average and drear subject . Since gravitational force is attractive , this should presumptively work to pull the universe together .
At the same time , several groups were observing removed supernova explosions . Supernovas ( Type Ia ’s , if you must be precise ) are really useful probes of the universe because a ) they are very bright , which imply that you may see them from very far aside , and b ) they are “ received candles ” which mean that if you realize them well enough , you may figure out precisely how brilliant they really are , and from that , you may set their space . As Idiscussed previously , we can measure the redshift of the supernova to tell how much the population has expanded since they boast up . From the combining of expanding universe and distance — voila ! We can determine how rapidly the universe is slowing down .
https://gizmodo.com/if-the-universe-is-expanding-whats-it-expanding-into-5526583
Only it is n’t .
In 1998 , theHigh - z Supernova Search Teamfollowed quickly by theSupernova Cosmology Projectannounced that based on their observation , the universe is , in fact , speed up . Subsequent notice have confirmed this , and the culprit has been dub “ Dark Energy . ” Like “ Dark Matter , ” the name is mean to obscure the fact that we have no frakking cue what it really is .
Dark energy , as you almost certainly know if you ’re an avid papa - sci referee , is a “ orphic substance ” ( it ’s always phone mysterious ) which causes the universe to quicken . This is not as ridiculous as it would seem at first rosiness , since when it come to gravity , you ’ve probably been lied to . You probably already know that hatful create a gravitational field , but general theory of relativity shows that any form of vim ( include a large box of photon ) will do the trick . Stranger still is that gravity gets an extra index - up if there is imperativeness involve . Under normal circumstances , we do n’t acknowledge this , since even in the center of the sun , the force per unit area is lilliputian compared to the vitality density .
Dark free energy is a weird event . The melodic theme is that the insistency is disconfirming — kind of like pliant — which think that the net gravity is abhorrent . This being io9 , I ’d be derelict if I did n’t point out that sullen energy is the closest thing that we have to anti - sombreness . It ’s not anti - gravity , take care you , but if you have your heart set on write it into your story , it ’s the good you ’re plump to do .
And there ’s a muckle of it ; our current good estimate is that dark get-up-and-go accounts for about 70 % of the total energy in the universe . Take a moment for that to slump in . For all of you who would give me grief about sinister matter simply because we ’ve never entrance a subatomic particle of the poppycock , consider the fact that at least dark matter has the good goodwill to comport something like everyday particles . With benighted energy , we have three times as much , and even our good mannequin do n’t involve particles that we might see in a detector . But that does n’t mean that we ’ve never seen anything like non-white energy .
Quantum Electrodynamics is , besides a potentially awesome name for a stripe , one of the most successful theory ever . It essentially commix electromagnetics and quantum shop mechanic , and has predicted everything from the elaborated social structure of the corpuscle to the magnetized strength of the electron to fantastic preciseness . It also has a well - earned bad reputation for grow lot of infinities in computation . This is forged , by and turgid , but we can ordinarily get around it by subtracting one infinity from another . Yes , it ’s a cheat . Yes , it makes me feel dirty . But it also works , and I guarantee that you ’re not hold up to make me sense any worse about myself than my quantum field possibility students did when I perturb them with puppies and quickly just erased the infinities .
One of the infinities that pop out of this theory is concern to the particles and antiparticles that are constantly being create out of “ the vacuum . ” There are two cool things and one really crappy thing about the “ vacuum energy ” of these irregular subatomic particle . The first cool matter is that it has exactly the negative pressure needed to make dark energy . The second cool thing is that it is n’t just made up . We can observe the“Casimir Effect”in a lab . Two metal plates in a vacuum will be draw together because there are more vacuum fluctuations outside the plate than between them .
The really crappy matter , however , is that any naturalistic calculation of the vacuum vigor gives an vigor denseness about 10100 meter larger than the density actually measured by the acceleration of supernovas ( even if you round down the “ eternity ” a mo ) . This is not a small problem . As far as I am concerned , it is the worst problem in physics , and one of the reasons that Donovan and other readers have amaze query trying to get around the whole affair .
One possibility is to intimate that the universe is n’t accelerating at all . Perhaps the most distant supernova are just different from the one near by , and that issue oddly piddle the population await like it ’s speed ? I had this thought myself , at first , but the supernova evidence is n’t the only reason to be a worshipper .
One of the best bit of evidence that we have is the measuring of theCosmic Microwave Background , the irradiation from when the creation was only about 380,000 yr quondam . We can abide by the bumps and wiggles of red-hot and dusty which behaved like water waves do today . Since the sparkle from these peak had to travel for almost the integral history of the universe before make our telescopes , we can use this information to visualise out the shape of the universe to incredible accuracy . It ’s “ plane , ” by the way . What this means for us is that since we know that black and ordinary issue combined only tally up to about 30 % of the Energy Department necessitate to make our universe matted , the rest must be made up by something else : dark energy . But that ’s not all . From the distribution of galaxies , to gravitative lensing , to the number of distant galaxy clusters , every piece of evidence points to a model with this coloured energy feature in it .
Basically , if you want to get rid of dark energy , you have to get disembarrass of relativity theory . You ’re welcome to try on , but for my money , it seems like a well stakes to endeavor to estimate out what glowering energy is .
The trouble is that we candidly do n’t have it off . One theory is that Einstein ’s “ cosmogonic invariable ” was right all along , and for some cause , it ’s simply firmly - wired into our theory of gravity . Another is that the dour vim is a fluid , with a pressure level that might exchange from place to place , and time to metre . So far , there ’s no grounds that this is the case , but the errorbars are still reasonably big .
But if the vacuum energy is so big , why is dark Energy Department so small ? We just do n’t know , which is why I got all defensive early on . One possibleness ( and this is a possibility probable to piss off many physicist ) is that it ’s simply the anthropic principle at work . perhaps all universes have more or less dingy vigor , but the I with more than us speed up so speedily that no stars or complex anatomical structure could form . This would mean , of trend , no stars , which means no human beings , orALFfor that matter , to be having this conversation .
So back to Donovan ’s original question : Could n’t the quickening of the universe be literal , but essentially be a pull signifier the exterior ? The problem is that the macrocosm does n’t have an exterior . Wherever you go , there you are . But I ’ll play along . Let ’s guess that our universe does have a crispy freshness , and some cosmic pizza - manufacturer is pulling it from all side . What go on then ? The edges may be accelerating forth from you , but the nearby mushrooms would more or less sit still . That ’s because gravity is a local curvature in distance - time , and it takes something — permit ’s call the dark energy yeast — to push all the mushrooms away . Hopefully the whole pizza pie digression will distract you from the fact that while physicists are passably bloody certain that something is playing the role of sullen vim , we have no idea what it is .
And now you sleep together our shameful , shameful arcanum .
Dave Goldbergis the author , with Jeff Blomquist , of“A User ’s usher to the Universe : Surviving the Perils of Black Holes , Time Paradoxes , and Quantum Uncertainty . ”(follow us onfacebookortwitter . ) He is an Associate Professor of Physics at Drexel University . Feel free to send e-mail to[email protected]with any question about the existence .
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