KH&CN nitdc ngoai I
KHUNG HOANG VE HANG SO VU TRU HUBBLE
Mpt bai toan da xuat hien doi vdi liang so Hubble. Hien co hai tap ket qua do dac dpc lap ve sU dan nd ciJa vu tru, cho chung ta hai cau tra Icii l(hac nhau. Trong thdi gian gan day lai xuat hien them ket qua do mdi
di/dng nhi/ la cau noi hai ket qua tren.
SI/khac nhau giQa cac ket qua do dang dan den tinh trang khung hoang doi vdi hang so Hubble. Vay dau la trj so chfnh xac? Phaichang cd nhUng bat cap trong thi/c nghiem? Vu tru se tien trien theo kich ban nao? Hay cdn khung hoang nay se phai din den mot vat ly mdi cua vu tru hpc ma cac nha thien van can phai tim kiem trong tiidng lai?
Hdng sa Nuimie
Dinh luat ciia Hubble du'dc coi la cd sd quan sat dau tien cho su md rong cija vu tru va ngay nay dong vai trb la mpt trong nhifng bang chiing thiidng dtJoc trich dan nhieu nhat de ho trg cho mo hinh Big Bang. Mac dii quan sat nay da so dugc quy cho Edwin Hubble, song khai niem vu tru gian nci vdi tdc do cd the tinh toan dugc lai ra doi lan dau tien vao nam 1922, tu cac phUdng trinh tudng
doi tong quat cua Alexander Fnedmann (hien difOc gpi la Phuong trinh Friedmann). Sau do, Georges LemaTtre, trong mot bai bao nam 1927, da chi ra rSng vu tru co the dang gian nd bang viec quan sat ty le giua van toe hoi quy va khoang each vdi cac vat the 6 xa, tiJ do de xuat mgt gia tn udc tinh cua hang sd ly le. Sau nay duoc sua bdi Hubble va dUdc goi
!a hang so Hubble.
HSng so Hubble = tdc do (km/sec/Mpc) dan nd
KHOAHOC
I KH&CN nudc ngoai
cua vu try = khoang each di chuyen trong mpt giay ciia cac mSt hinh lap phUdng co canh la 3.26 trieu nam anh sang (Mpe • megaparsecs).
Cac Uch bin d6n uitn cia vn tra
Hon 13,7 ty nSm trUdc da xay ra vu no lon (Big Bang) tu mpt plasma sieu mat do. Sau do ta qua trinh dan nd roi nguoi dan: nguyen tCi, khi. sao- b^t d^u hmh thanh. TU mo hinh Big Bang, cac nha khoa hpc da dua ra nhieu kich ban cho tUdng lai ciia vu try;
- Big Chill (Ddng lanh ldn): vu try ludn dan nd va ng^y cang lanh gia.
- Big Crunch (Co Igi Idn): qua trinh dan nd dudc thay the bdi qua trinh co lai. Moi vat bien thanh mdt dilm ky dj.
- Big Bounce (Nay bat ldn): vu tru theo chu ky se CO Igi roi dan nd Iren ddng tro cua minh.
- Vu try dn djnh: sU dan nd vu tru se cham dUt.
- Big Rip (Xe rach Idn): vu try ngay cang dan nd va se b! xe rach.
Khiing tioang cua h g sA vu tru Hubble
De do h^ng sd Hubble, cac nha thien van sCf dyng nhQng sao ggi ia "nhQng nggn nen chuan" - la nhUng sao ma ho biet rd khoang each va dd sang npi tai (intrinsec brightness). Cac sao chuan mo dan khi di xa (vi du: mgt bdng den 60 W se xuat hien md dan khi ban di chuydn ra xa nd, nhung neu ban biet dd la bdng den 60 W, ban cd the suy ra sy tach biel cua nd vdi ban), dieu nay giup chung ta xac dinh dUdc hang sd Hubble,
Ket qua do trudc day (tren co sd cac dQ lieu vu tru nguyen thiiy) cho sd 67,4 km/giay/megaparsec, dieu dd cd nghia la cac thien the thien van lui xa dan khdi Chung ta vdi van tdc 67,4 km Iren mdi khoang each nam anh sang.
Nam 1998 (tren CO sddQ lieu vu tru hien dai) Adam Riess (Giai Nobel vat ly 2011) thu duoc mdt ket qua do hang sd Hubble la 74 km-giay-'megapersec (lon hon 9°o so VOI 87.4) • hmh 1 Trong phuang phap do
Hinh 2. Biii toan hang
cua minh, Riess da sif dung cac sao Cepheids' va nhUng sieu tan tinh la (Type l a supernovae)^ nhU nhOng sao chuan. Tri sd 74 cua hang sd Hubble do Adam Riess do dUdc kha trung vdi nhOng ket qua dya tren cd sd dU lieu mdi (sif dung quasars (73,3 va 74,2) va masers^ (73,9) - hinh 2.
Nhu vay, khi cac nha vu tru hoc tinh trj so ciia hang sd Huble tren cd sd cac dO lieu cua vu try nguyen thiiy hoc hp da lim ra mpt tri so nhd hdn so vdi cac kel qua thu dUdc tren cd sd cac dU lieu ciia vu Iru hien tai.
"Cepheids la nhflng pun-xa (sao neutron) khong 16 c6 6b sang thay doi Iheo chu ky, giOa chu ky va <3b sang co mot he thflc d§C trUng:
chu ky cang dai thi dp sang cang ldn, nhu vay chu ky quan sal dfldc la chi so do khoang each cua sao. vi vay cac Cepheids dong vai tro quan Irong trong viec xac dinh cac khoang each (Cepheids dflOC Leawitl phat hien nSm 1912)
Sieu Ian tmh (supernovae) la nhflng sao d d i nhic-r •-. ijog no Ird nen -uc sang manh li^t, co hai loai sieu tan linh (1 va ll( ,',;. i r^ (JA ^^j^g nh§i va Ihudc he sao ddi, ehfl nova cd nghTa la T,',
Masers la hieu ung luong tu laser irong c^c l h > f • ; r. ^^ y,^.
dmh khoang each
62
KHMHOC \ . \ N m c NGHE \e; NamKH&CN nUdc ngoai I
Hinh 3. Sao Antares 6 trung tam hinh tren la mot sao do sieu khong lo a cuoi db'i.
Tuy nhien gan day, lac gia F r e e d m a n (Dai hgc Chicago, My) chp r a n g , cac C e p h e i d s khdng phai la nhUng sap chuan lot vi cac sao nay bien ddi Iheo tuoi, hdn nUa c h u n g iai d t r o n g nhOng v u n g day bui thien van lam m d bQc tranh tdng the, dSn den lam xa khocing each cac sao. Ben c a n h d d . tinh chat c y m nhau lai ciia cac sao nay lai lam cho c h u n g d u d n g nhu d m p l k h o a n g each gan h o n . Vi the F r e e d m a n da sd dyng cac sao T R G B (Tip of Red Giant Branch - cac sao dau tren N h a n h sao k h d n g Id d d ) .
T R G B la nhUng sao khdng Id do cd d d s a n g maximum gidng n h u n h a u lam c h o T R G B thda m a n cac dieu kien sao c h u a n . C h u n g lai nam trong nhUng vung khdng c d bui thien v a n , khdng chua nhieu cum sao ( n h u trong Magellanic C l o u d va thien ha IC 1613). NhDng dieu kien tren cho p h e p F r e e d m a n do duo'c hSng sd H u b b l e la 69,8+0,8, vdi d d chinh xac 10%. Hien nay nhieu nha thien v a n c h o rang cac ket qua cua Freedman ia d a n g tin cay. V a neu h a n g s d Hubble > 71-^•72 th) vu try se tien t h e n trong v d n g 20 ty nam theo m p l kich ban khde liet la kich ban Big Rip.
Kei luan
Sy di biet c u a cac tri s d H u b b l e dang gay nen mpt tinh Irang "eang thang H u b b l e "
trong thien v a n hpc. Ngudi ta khdng hidu dUPe nhQng bat cap nao trong thuc nghiem da dan den di biet d d . T r o n g nSm 2 0 1 9 , cac nha thien vSn dUa ra t h e m nhieu s u dj biet khac, lam rdi them linh trang nay D e n thang 11/2019 linh trang c a n g thang ldn tdi mUc nha vat ly J o s e p h Silk p h ^ i p h a l bieu rang: day quci that la mdt cuoc "khiing hoang ciia vu try hgc".
Hien nay, cac nha val ly van d a n g dau dau vdi nhQng eau hdi: dau la tri sd chinh xac? C d Chang nhQng bat cap trong thuc n g h i e m ? Vu try se tien t n e n theo kich ban nao? Lieu c d n khung hodng nay ed dan den mdt vat ly mdi eiia vu tru hpc ma cac nha thien van can phai tim kiem trong lUdng l a i ^
TAI UEU THAM KHAO
1. Natalie Wolchover (2020), New Wrinkle Added to Cosmology's Hubble Crisis, https,//www,quantamagazine.
o r g / n e w - w r i n k l e - a d d e d - l o - c o s m o l o g y s - h u b b l e - crJsis-20200226/
2 Natalie V\/olchover (2019), Cosmologists Debate How Fast ihe Universe is Expanding, hnps.//www.
quaniamagazine.org/cosmologists-debatG-how-fasl-lhe- universe-is-expanding-20190808/
3. Richard Panek (2020), "A cosmic crisis", Scientific American. 322(3), pp.30-37,
4. https , vi.wikipedia.org/wiki/%C4%90%E1%BB%8 BnhJu'^=E1=oBA''=ADt_Hubble,
KHOAHOC \ ' l^i 63
