AK­SHIT SAN­GOMLA ABRA­HAM LOEB,

Down to Earth - - Palette / Interview - @aks7489

All ob­servers agree that the ex­pan­sion of the uni­verse is ac­cel­er­at­ing (ow­ing to “dark en­ergy”, whose na­ture we do not un­der­stand, but which rep­re­sents the en­ergy of vac­uum with­out mat­ter). But there is a de­bate re­gard­ing the ex­pan­sion rate. We can in­fer the ex­pan­sion rate that the uni­verse should have to­day based on the data we have on the cos­mic mi­crowave back­ground

[CMB is an elu­sive ra­di­a­tion that em­anated at or af­ter the birth of the uni­verse and ex­trap­o­lat­ing its tra­jec­tory to present time is one of the ways to cal­cu­late the ex­pan­sion of the uni­verse]. But some ob­servers who mea­sure the ac­tual ex­pan­sion rate to­day ar­gue that their mea­sured value dis­agrees with the ex­pected value at a sta­tis­ti­cally sig­nif­i­cant level.

Bang) and to­day. We do not know if that is the case. [Sci­en­tists es­ti­mate that 27 per cent of the uni­verse is dark mat­ter which does not ab­sorb, emit or re­flect light and whose ex­is­tence is in­ferred only from the grav­i­ta­tional ef­fect it seems to ex­ert on vis­i­ble mat­ter.]

We have ev­i­dence that there is much more mat­ter out there than the or­di­nary mat­ter we are made of. First, the in­ho­mo­geneities at early times would have been smoothed out by the ra­di­a­tion if there was only or­di­nary mat­ter. There needs to be a type of mat­ter that does not cou­ple to the ra­di­a­tion in or­der for gal­ax­ies like the Milky Way to form. Sec­ond, when we look at gal­ax­ies, we in­fer that they must con­tain much more mass than the vis­i­ble mass of their gas and stars. This was known for 70 years, since Fritz Zwicky in­ferred that clus­ters of gal­ax­ies con­tain much more mat­ter than their vis­i­ble mass. But we still have no clue as to the na­ture of that dark mat­ter. It is most likely made of par­ti­cles that do not cou­ple to light (this be­ing the rea­son that we can­not see them), but we do not know what par­ti­cles these are.

The break­through in our un­der­stand­ing could come from lab­o­ra­tory ex­per­i­ments. There were hopes that new par­ti­cles will be pro­duced and dis­cov­ered at the Large Hadron Col­lider or other ex­per­i­ments, but so far we have had no suc­cess.

The Big Bang model is sup­ported by a large body of ev­i­dence. It pos­tu­lates that the uni­verse started from a hot dense state, and we have de­tected the relic ra­di­a­tion left over from that state. The model also as­sumes that the ini­tial state was nearly uni­form with small in­ho­mo­geneities that grew over time due to the at­trac­tive force of grav­ity to make the struc­ture we see to­day in the form of gal­ax­ies and stars, of which the Milky Way host­ing our Sun are ex­am­ples. In­deed, we find the cos­mic mi­crowave back­ground to have al­most ex­actly the same bright­ness in all di­rec­tions in the sky with small vari­a­tions of the ap­pro­pri­ate mag­ni­tude, re­flect­ing the ex­pected ini­tial state. The model also pre­dicts the abun­dances of light el­e­ments, like he­lium, deu­terium and lithium, which were cooked in the first few min­utes of the cos­mic ex­pan­sion af­ter the Big Bang (when the uni­verse as a whole was hot­ter than the in­te­rior of stars). The pre­dicted abun­dances agree with ob­ser­va­tions. Al­to­gether, the data we have pro­vides ro­bust sup­port to the Big Bang model but it also leads to in­trigu­ing ques­tions:

(i) What led to the Big Bang? In the very first in­stants, quan­tum me­chan­ics was as im­por­tant as grav­ity, but we still do not have a the­ory that uni­fies these two pil­lars of mod­ern physics and so we can­not pre­dict what may have hap­pened be­fore the Big Bang.

(ii) We in­fer from cos­mo­log­i­cal data that most of the mat­ter and en­ergy in the present-day uni­verse are dark. We call them “dark mat­ter” and “dark en­ergy”, but these are just la­bels that sig­nify our ig­no­rance.

With­out hu­mans: in a few bil­lion years, the Milky Way will col­lide with its near­est neigh­bour, the An­dromeda gal­axy and the night sky will change. In about seven bil­lion years, the Sun will die. First it will ex­pand to a red gi­ant and pos­si­bly en­gulf the Earth and then its core will cool and con­tract to make a “white dwarf”, a piece of dense metal the size of the Earth. Most of the stars have a mass that is 10 times lower than that of the Sun and they will con­tinue to shine for up to 10 tril­lion years (1,000 times longer than the Sun). Af­ter that, there will be dark­ness. If the

MOST OF THE MAT­TER AND EN­ERGY IN THE PRESENT-DAY UNI­VERSE ARE DARK. WE CALL THEM “DARK MAT­TER” AND “DARK EN­ERGY”, BUT THESE ARE JUST LA­BELS THAT SIG­NIFY OUR IG­NO­RANCE

ac­cel­er­ated ex­pan­sion of the uni­verse con­tin­ues, it will be­come a dark and lonely place with our gal­axy (the merger prod­uct of the Milky Way and An­dromeda) sur­rounded by vac­uum.

With hu­mans: since tech­nol­ogy evolves ex­po­nen­tially with a time con­stant of a few years, we will wit­ness vast ad­vances in Ar­ti­fi­cial In­tel­li­gence (AI), ro­bot­ics, and ge­net­ics. Within a thou­sand years, hu­mans will build ma­chines that tran­scend them and can ven­ture into a long jour­ney into space. 3D print­ers equipped with AI will pro­duce life as we know it on other plan­ets out of the raw ma­te­ri­als there. We might also find ev­i­dence for other civil­i­sa­tions that are far more ad­vanced than we are.

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