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The universe is expanding . But depend on where we look , it ’s doing so at bafflingly unlike speed .
The job is known as the Hubble tension , and it centers around figuring out a number for the universe ’s expansion charge per unit , called the Hubble invariable . To ascertain it , scientists have pore over tiny fluctuations in the cosmic microwave scope ( CMB ) — an ancient souvenir of the universe ’s first light — and built cosmic length ladders to remote , pulsating whiz call Cepheid variables .
An artist’s illustration of the Big Bang.
But the best experiments using these two method disagree . The difference in results may have seemed small , but it was enough tospark a major crisis in cosmology .
Wendy Freedman , an astrophysicist at the University of Chicago , has spent four ten studying the Hubble constant .
Now , she is using one of the most muscular tool in uranology — theJames Webb Space Telescope(JWST ) — to get the most exact measurements yet of the Hubble invariable . Her squad is looking at several heavenly object at the same aloofness from Earth . The hope is with several measuring in paw , the tenseness can finally be resolve one way or another .
Wendy Freedman.
Live Science spoke to Freedman about how the tension arises , why it matters , and how she ’s using the JWST to trace for an solvent .
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Ben Turner : You’ve been measuring the Hubble constant for a turgid part of your scientific career . What drew you to studying it ? And why is it such an significant mensuration for cosmologists to know ?
The period-luminosity relationship used to measure the distances of Cepheid stars both in the Milky Way and the neighboring Large Magellanic Cloud, as captured by NASA’s Spitzer Space Telescope.
Wendy Freedman : The Hubble constant gives you a measure of the size of the universe of discourse , and it ’s probably the most fundamental parameter that we can valuate that tells us about the evolution of the universe .
What suck me to it is the fact that you could make measurements in our local neighborhood — which , of course , astronomically speak , is a large neck of the woods — and use them to learn thing about the early universe and how it get . It really intrigued me .
BT : How authoritative is the stock model of cosmology ? What is the Hubble tautness ?
The cosmic microwave background: The universe’s ‘baby picture’ taken by the European Space Agency’s Planck satellite
WF : The standard good example [ which explains how the universe has expatiate since the Big Bang ] is an interesting model in the sensation that we are made of what is a very small fraction of the overall amount of matter and vim in the universe .
And so there are very fundamental thing that we do n’t understand . We do n’t jazz yet what dark matter is . Neither do we experience what dismal energy is , except that it ’s causing the universe to speed up . But the manakin works signally well , return that we do n’t understand its fundamental structure .
The Hubble constant gives us an opportunity to determine something more about the universe in that way . We essay the stock model by making mensuration locally , and then equate them with what we find in the other cosmos by measuring fluctuations in temperature across the cosmic microwave background .
you could fit the received model to those cosmic microwave background signal measurements , and it ’s an astoundingly good conniption . And because the standard model is a predictive model you could work forwards , using data from the cosmic background radiation to forebode what the Hubble invariable should be today .
But if we compare that predicted value of the Hubble invariable to what we appraise using star topology called Cepheid variables , they do n’t match — that ’s the Hubble tension .
BT : If we live with that the Hubble tension is real and not a taxonomical fault somehow , how big a challenge is it to the standard exemplar of cosmogeny ?
WF : At this full stop I have a completely open head [ on whether it ’s real ] . I do n’t know which means this is go to go . But yes , it would be pregnant . How substantial ? Probably not as significant as the standard model itself . But if it lead to a New , fundamental understanding that improve our knowledge of these thing that really remain mysteries at the consequence , it could be profound .
BT : So let ’s dig into how we measure this . Besides the wavering in the cosmic microwave oven background , Cepheid variables are the other main way astronomers find a value for the Hubble constant quantity . What are Cepheid variables , and how do we use them to measure astronomic distances ?
WF : Cepheid variable were what Edwin Hubble used when he happen upon the elaboration of the universe . They ’re stars that are five to 20 time more massive than our own sun , and they have atmospheres that are in reality pulsating — moving in and out — with prison term . They do this in a very regular way for periods of a couple of days , going through up to 100 or so cycle in their light levels .
In the other 1900s , Henrietta Leavitt found that there was a correlation between how fast Cepheid stars were pulsating and how lustrous they are . That gives us a means of measuring distance , and it ’s one of the most accurate mean that astronomers have today .
If we can evaluate nearby principal in a way that we can determine their distance , say from geometry . Then we can count at Cepheid variables in galaxies , equate their brightness at a given period — using the menstruum luminance relation — and then by the inverse square jurisprudence of lighting [ light dims from a source in balance to the square of the distance to its viewer ] we get the aloofness .
BT : And yet despite being very precise , there are a lot of uncertainties associated with Cepheid measurements . What are they ? And what are researchers doing to describe for them in their measurements ?
WF : There are complications . There ’s rubble between us and the Cepheids that make them dimmer ; their atmospheres contain dissimilar amounts of intemperate elements that can change the luminousness [ meaning they have a high metallicity ] ; and there are just dubiety in the measurements .
Also when we go to more distant galaxies , it ’s very difficult to make a measuring of a Cepheid on its own because other stars in the galaxy contribute luminousness that ’s hard to separate from the Cepheid itself .
We ’ve been improve the accuracy of these measurements for ten . Before the turn of the century , we were arguing about Hubble constant from Cepheids being between 50 to 100 [ km per second per megaparsec ] — literally a gene of two uncertainty . As of 2001 , our group write a result that chip in a value of 72 [ km / s / Mpc ] with 10 % uncertainty . That value stick out the exam of time : If we regard Cepheids today we get numbers like 72 , 73 and 74 .
BT : But when we look at late measurements of the cosmic microwave ground taken by the Planck planet , we get a value of around 67 . At first glance , that look like a difference of at most 7 km / s / Mpc , peradventure even less . At a passing glance , that ’s not very handsome , so why does it matter ?
WF : Where the tensity has arisen is that , in the last several age , it ’s been possible to make really precise measurements of small differences in temperature in the cosmic microwave oven setting . We ’re talking really minuscule — like a one-thousandth of a percent .
You mensurate these fluctuations accurately and you’re able to fit the standard example of cosmogony incredibly well to this spectrum of temperature remainder . From that , you may deduct that the Hubble constant is 67 .
Now there seems to be this disagreement between 67 and 73 . That does n’t sound like a lot given that we started between 50 and 100 . In fact , Hubble commence off at 500 when he first made his measurement . But because the measuring are improving in their truth , it appears as if it might be quite significant .
BT : So how are you looking for an response ?
WF : Why I ’m excited decently now is because we have the chance with the James Webb Space Telescope to make measurements of Cepheids and also other kinds of stars .
We ’ve speak about systematic errors from rubble and metallicity and so on . Each method we ’re live on to use is going to have its own bent of systematic doubt . No matter how many times we make the measurement more accurate — those systematics are going to get you in the last if you do n’t understand what they are .
So what we did in the past was take precise mensuration of stars at the summit of the red giant leg [ which also pulsate regularly ] as a comparability . We got outcome for that come in at around 70 . Within their uncertainties they agreed pretty well with the Cepheids , but they also agreed pretty well with the cosmic microwave ground .
Our current JWST computer program is to measure the Cepheids , tip of the red-faced elephantine branch stars and a third star known as a JAGB wizard [ aging carbon mavin with a near - constant cleverness ] in the same galaxy , all at one distance . We ’ll see how well we agree and that will give us a sense of an overall systematic answer .
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BT : Very briefly , why are bakshis of the red-faced giant branch stars a utile comparing to make with the Cepheids ?
WF : They’re quondam wiz or low aggregative star — they do n’t have much of a metallicity dependence . We do n’t realize the metallicity dependence of the Cepheids well , that ’s still something that persist unsolved .
Also Cepheids are young , so they have n’t had time to fan out away from the regions where they were formed . They ’re in crowded high surface tightness realm , whereas the red giants are isolated . So it ’s very bare to make a measurement in terminal figure of their brightness level .
BT : Are there any results you could loosen ? How shortly will you get them ?
WF : Not yet , our mathematical group right now is blind so we ’re not go to do an absolute calibration in the aloofness airfield until we have all the data point measured and analyzed . We have to measure the periods and luminance of the Cepheids , create a catamenia - luminosity relation and ( along with the JAGB wiz ) measure these luminosities . We ’re not go to unblind until all that analysis is done . We ’ll sit down in a room and we ’ll know .
So I do n’t have it off in damage of the absolute [ distance ] calibration . But what I can say , about our database and the reason we put in this bighearted marriage proposal to utilise JWST , is that it ’s experience four time the resolution of theHubble Space Telescopeat infrared wavelengths . This means the star crowd together issue is palliate hugely and we have a test using a different filter to look for metallicity effect directly where we ’re observing . So I think we ’re going to be able to get at many of these systematic effects .
Where the Hubble constant is pass away to fall from this I just do n’t bang right on now . But we ’re really worked up because I think we ’re gon na have something really interesting to say . In our first galaxy we see a lot of difference from the Hubble [ Space Telescope ] measurements — those stars were really crowded . Now we ’re look at galaxies that are not quite as crowded .
As I said , I ’m just completely open . I do n’t know where this is going to descend . But it is a enquiry . It ’s an empiric enquiry .
