EDUARDO SOTERAS/AFP/GETTY

now vindicate Planck’s findings and producea very similar value for the Hubble constant.
But neither result matches direct
measurements of the Hubble constant — adiscrepancy that has become known as theHubble-constant tension. Astronomers whouse the brightness of particular stars and supernova explosions, collectively called standard
candles, to calculate the expansion rate findthat galaxies rush away from each other roughly
10% faster than CMB maps predict.
Many researchers had hoped that, astechniques became more accurate, the gap
would shrink. Instead, narrowing error bars
for each type of study have only made theinconsistency more significant.
The ACT is the first ground-based CMB
experiment that could have challenged Planck’sresults, says Erminia Calabrese, a cosmologist
at Cardiff University, UK, who led the analysisof the data. The telescope’s design and location,
just inside the tropics, enables it to map moreof the CMB sky than can other ground-based orballoon-borne telescopes, which have typically
been limited to smaller regions.
Mapping the sky on a large scale is crucial
for calculating the key parameters of cosmic
expansion, Calabrese says. Another strengthof the ACT was that an upgrade in 2013 allowed
it to make precise measurements of the polarization of the CMB radiation, says principal
investigator Suzanne Staggs at Princeton
University in New Jersey. Polarization data
reveal the effect of galaxies in the foregroundon how the CMB travels, and help to make thecosmological measurements more precise.
“For the first time, we have two data setsmeasured independently and with enoughprecision to make a comparison,” Calabresesays. Having also been a member of the Planck
team, she says it was a relief to find that the two
experiments’ Hubble-constant predictions
agreed to within 0.3%.
This agreement between ACT and Planck is“a truly major milestone”, says Paul Steinhardt,
a theoretical physicist at Princeton University.
“I am very impressed by the quality of the newdata and their analysis,” he adds.
Adam Riess, an astronomer at Johns Hopkins
University in Baltimore, Maryland, who has led
much of the cutting-edge work on standard
candles, says that the ACT data’s agreementwith Planck is “reassuring” and “a testament
to the quality of the experimenters’ work andcarefulness”.
But the tension over the Hubble constantremains. Techniques developed by several
teams could help to resolve it. Steinhardt
thinks that the measurements will eventuallyconverge as experimentalists perfect their
methods.
But Riess says that perhaps it is cosmology’s
standard model that is wrong instead. “My gut
feeling is that there’s something interesting
going on.”Ethiopia’s Grand Renaissance Dam is two-thirds built.

ROW OVER GIANT NILEDAM COULD ESCALATE,EXPERTS WARN
Ethiopia wants to start filling the dam’s reservoir thissummer. Egypt calls the project an ‘existential threat’.
By Antoaneta Roussi

Researchers are warning that Egypt,
Ethiopia and Sudan need to movefaster to resolve a long-running dispute over the building of Africa’slargest hydroelectric dam.
Seasonal rains are starting to fill the reservoirof the Grand Ethiopian Renaissance Dam, set
to become Africa’s largest hydroelectric power
plant, on the Blue Nile river. Two-thirds of thedam has been built.
Egypt’s government, which has opposed theproject since it began in 2011, calls the dam an‘existential threat’. It is concerned that the dam
will reduce the nation’s water supplies, whichcome almost entirely from the Nile, particularly during times of drought.
Ethiopia, by contrast, calls the dam an‘existential necessity’. Its citizens — whose taxes
mostly paid for the dam’s nearly US$5-billion
price tag — are anticipating electric power, aboost for industry and new jobs. The WorldBank estimates that nearly half the country’s
population lacks access to electricity.
According to hydrologists and politicalscientists, already-strained relations between
Ethiopia, Egypt and Sudan could worsen if adeal isn’t quickly reached.

South Africa’s President Cyril Ramaphosahas been hosting talks on behalf of the African
Union, and the three nations have resolved
some key issues, including the volume of waterand time needed to complete the fill. But thelatest talks ended without agreement on 13 July.
There is still an impasse over what would
happen in the event of a drought, as well assome other technical and legal issues. Ethiopia
is keen for reservoir filling to begin properly
during its rainy season of July and August.
From Ethiopia’s perspective, if it misses
the summer window, it would have to wait
another year to start filling.
In a normal-to-wet year, Ethiopia, Egypt andSudan have effectively agreed that, after an initial 2 years of filling, the dam’s reservoir would
reach 18 billion cubic metres (b.c.m.). After
that, Ethiopia would retain around 10 b.c.m.
each year to operate electric power from thedam if conditions are normal to wet, researchers familiar with the talks have told Nature.
In the case of a drought year, the filling
period would extend to seven years. However,
the three sides have yet to agree on what to doin this case — one of the sticking points in thecurrent negotiations.
According to Kevin Wheeler at the Environmental Change Institute at the University of.
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Oxford, UK, there is no single standard definition of ‘drought’. But the countries have agreed
that when the flow of Nile water to the dam falls
below 35–40 b.c.m. per year, that would constitute a drought. In such an event, Egypt andSudan want Ethiopia to release some of thewater stored in the dam’s reservoir.
Representatives of both countries say thiswould still allow Ethiopia to continue generating electricity. But Ethiopia prefers having
the flexibility to decide how much water torelease during drought conditions, because
more water equates to more power per unit ofwater. The country also wants to reduce the riskof running the dam’s reservoir to low levels.

Law in action
The agreement’s legal status and how disputes
will be resolved are other impasses. Egypt wants
any final agreement to have the status of aninternational treaty. It also wants a third party,
such as the African Union or the United Nations,
to resolve any disputes. Ethiopia prefers disagreements to be settled between the riparianstates, with no foreign parties involved.
Egypt and Ethiopia do not have a formal
water-sharing agreement. Under the 1959
Nile Waters Agreement between Egypt andSudan, Egypt takes 55.5 b.c.m. of water fromthe Nile each year, and Sudan takes 18.5 b.c.m.
That agreement was reached shortly before
Egypt began constructing its own mega dam,
the Aswan High Dam. Ethiopia, however, wasnot part of this agreement and therefore does
not recognize it.
Ashok Swain, who studies peace and conflictat Uppsala University in Sweden, says that thecountries might decide to sign a short-term
agreement, perhaps lasting a year, if they areunable to agree on all outstanding issues.
“There have been several examples of
one-year agreements in these kinds of treaties, like the 1975 one of India and Bangladeshon the Ganges,” he says. “The two countries
couldn’t agree how to operate, so they started
with one year first and then extended to 3, 5
and then 30 years, which I think could be thecase [here] as it will buy both sides some time.”
Mohamed Fouad, a member of the Egyptian
parliament, told Nature that if Ethiopia needs
power, then involving a third party, such as
the World Bank, in financing Ethiopian power
stations could help to break the stalemate.
Egypt’s water minister Mohamed Abdel Aty
has proposed that Egypt could potentially shareelectricity with Ethiopia, similar to its arrangements with other countries such as Sudan. Sucha proposal would mean that Ethiopia has accessto energy and so could slow the filling of thedam until the nations reach an agreement,
Fouad says. “One nation’s need for electricity
is pinned to another nation’s need for water.”
Swain says a military confrontation isunlikely, but that goodwill between thecountries is in short supply.

PANDEMIC SPEEDS MAJORTEST OF UNIVERSALBASIC INCOME
Economists welcome chance to see whether giving
people no-strings cash can improve livelihoods.
By Carrie ArnoldSpain’s government has started whatmight just be remembered as theworld’s biggest economics experiment. On 15 June, spurred by thecoronavirus crisis and its economic
fallout, it launched a website offering monthly
payments of up to €1,015 (US$1,145) to thenation’s poorest families.
The programme, which will support
850,000 households, is the largest test yet of
an idea called universal basic income (UBI) — inwhich people are given a cash payment eachmonth to spend however they choose. It hasoften been discussed, but never satisfactorily
tested, and economists around the world arewatching closely to see what the impact of thescheme on livelihoods will be.
The move comes at a time of unprecedented
economic turmoil brought on by the coronavirus pandemic. Spain was one of the hardest-hit
countries in the early days of the outbreak. The
nationwide lockdown curbed the spread ofthe virus, but came at a staggering financialcost. Millions of people lost their jobs as theeconomy shrank rapidly, putting many of themost vulnerable citizens at risk.
“If there’s ever an opportunity to try topush for some sort of income floor that can.
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Volunteers in Barcelona, Spain, prepare food packages for vulnerable people.

be paid out in cash to people, this is the time
to do it,” says Damon Jones, an economist at
the University of Chicago in Illinois.
Even before the coronavirus struck, thecountry’s left-leaning coalition government
had proposed the scheme — a variant of UBI
called guaranteed minimum income — but
the resulting economic emergency brought
the timeline forward. The system will allocate
a fixed monthly sum to each eligible household — no strings attached. The aim is to provide recipients with enough cash to meet theirbasic needs without trapping them in poverty inthe same way as existing welfare programmesthat offer support only to those without jobs
or other income, says Spain’s social security
minister, José Luis Escrivá.
He estimates that the scheme will cost thegovernment at least €3 billion per year. “Thereis a huge interest in Spain for this,” he says. Thewebsite where people can apply for grants
launched on 15 June and received more than
50,000 applications in the first 4 hours.
Several other countries have experimentedwith UBI, but until now, most trials have been
limited to a few hundred or thousand people.
Spain’s scheme — which was passed by theCabinet on 29 May — is the first to be rolled out
nationwide. For researchers, it could provide
a real-world opportunity to study the impacts