NHlN Rfl T H ^ Gidl
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Hap din luvng tir -
BigBounce thay vi BigBang
Trong vdt ly hien dai cd 2 ly thuye't tru cot la Ly thuyg't lircrng tur va Ly thuyi't hdp din.
Can phai thong nhd't 2 ly thuye't dd. Ddy Id mgt cdng vi$c c^c ky khd khdn. Hifn nay co hai ly thuyg't quan trong dang tiS'n hdnh cdng cupc thong nhat do: Ly thuyet day (LTD) va Ly thuyet Hdp din lirtfng td vdng (Loop Quantum Gravity LQG). Bai viet gidi thieu vi Ly thuyet LQG din den nhi6u didu mdi me nhir thay vi BigBang ta phai co BigBounce (Vu nay bat Idn) cua vu tru trong qud khd.
LTD CO tham vpng Idn hPn Id thd'ng nhd't cd 4 tOPng tdc: didn tO, ydu, mgnh vd hap ddn. Song LTD Igi sO dung mdt khdng thdi gian nhd't djnh, ndi mdt each hlnh tOdng Id ldm ddng eOng {freeze) khdng thdi gian Igi, didu nay lam cho tfnh tong quat cua ly thuyd't bi md't di, ngodi ra LTD khdng dOa ra dope mdt thf nghidm ndo d l kilm chOng Igi ly thuydt.
LQG ed mdt muc tidu hgn ehe hdn Id ch? nhdm thd'ng nha't hap ddn vd lOpng tO, song day Id mdt bode dau quan trpng. LQG dang cd nhidu t h i n vpng vl khdng sO dung mdt khdng thdi gian nhat dmh nao ca ma khdng thdi gian phai dope quyet dmh bdi vat chat nam trong dd [1], [2], [3] va [4].
Hinh 1: td trii sang phii: Abhay Ashtekar, Carlo Rovelli. Lee Smolin
n:
Ngodi dau tien cd cdng trong viec kien thiet LQG la Abhay Ashtekar, mdt nha vdt ly gd'c An, thudc Dai hpc Syracuse. Thay vl sO dung methc khdng thdi
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gian, Ashtekar dd nghi sO dgng dgi lopng gpi la lidn thdng (connection) A ddng vai trd nho thd 4 chieu
(4- potential) trong didn tO.
Tid'p theo, nhOng nhd vdt ly cd cdng nhidu nhd't trong xay dong LQG Id Lee Smolin vd Caho Rovelli (hlnh 1). Danh tO "vdng" (loop) phdt sinh tO vide mdt sd tfnh todn trong LQG gan lien vdi nhOng vdng ve ndn trong khdng thdi gian (nho hdm chuydn song song parallel propagator eua A dpc theo vdng kfn).
LQG doa trdn 2 nguydn tac: 1) Hlnh hpc eua vu try khdng Id mdt didu tidn quyd't (khde vdi LTD dope xdy dong trdn mdt hinh hpc djnh trode); 2) Ly thuyd't phdi bd't bidn ddng ed'u (diffeomorphism), didu dd cd nghTa Id chung ta cd quydn lOa chpn bat ky mdt he tpa dp ndo d l dnh xg khdng thdi gian vd cdc phOPng trinh (ndi cdch khde khdng ed he tpa dp ndo Id dae bidt cd).
Khong - thdi gian khong con la lien tuc
Trong LQG, khdng thdi gian khdng cdn Id lidn tuc nOa md dope ed'u tgo bang nhOng "nguydn tO khdng thdi gian", dd'y Id nhOng phan tO t h i tfch khdng thdi gian khdng phdn chia dope nOa (hlnh 2). Nho vdy, vu trij khdng bien thidn mdt cdch lidn tuc md tid'n t h i n tOng bode nhO trong mdt mdy tfnh.
6 mat bao quanh). Mdt hlnh chdp day vudng dgt chdng ldn mdt hlnh lap phOdng {hinh 3c, 3d) se Ong vdi mgng spin cd 2 nut (bieu didn t h i tfch cua hinh chdp vd hlnh ldp phOdng) va 10 dOdng (bieu didn 9 mdt eua hinh tong hpp, vd mdt dOdng ndi 2 d?nh Ong vdi mat ehung eua hlnh chdp va hlnh vudng).
Ndi ehung, trdn mgng spin mdt lOpng tO eua didn tfch dope bilu didn bdi mdt dddng, cdn mdt lOpng tO cua t h i tfch se dope bilu dien bdi mdt nut. Con sd tgi nut ch? t h i tfch (dPn vi /^^vdi ip la dp ddi Planck), cdn con sd trdn ede dodng eh? didn tfch (dPn vi /p^.
Hlnh 2: ly thuydt tuang d6i g$p khd khan vl cho ring, khdng - thdi gian lit mdt continuum (da t^p lidn tuc). LOG l$i cho rkng, khdng thdi gian la mdt mang ludi khdng lidn tuc cCia cic "nguydn td"
khdng - thdi gian (cic hlnh ciu trdn hinh ve). DUdng kfnh cija cic hlnh ciu dd bang dd dai Planck, d vung kfch thUdc dd cUdng dd luc hip din ngang bang vdi cic hidu dng lU0ng td
Trong LQG, ngOdi ta sO dung mdt mgng spin (spin network) d l bilu didn cdc trgng thdi lOpng tii cua khdng gian 6 vung vi md (hlnh 3 vd 4). Vf du, mdt hinh lap phOPng gdm mdt t h i tfch bao bpc bdi 6 mdt hlnh vudng se Ong vdi mgng spin {hinh 3a, 3b) CO mdt nut (bilu dien t h i tich) vd 6 dOdng (bilu dien
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Hlnh 3: dien tich, the tich va bieu didn tren m^ng spin
Hinh 4: hlnh ve nghd thuat cua mdt mau mang spin
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Mgng sd dT cd ten Id m^ng spin vi cae dOdng trong (ndi lien 2 d?nh) eua mgng dope gdn lien vdi mpt bilu didn bd't khd quy cua mdt nhdm Lie compact (Ong vdi spin). Mgng cdc dodng ndi lidn cdc d?nh lam thdnh mdt dd thi (graph) nhO dd thj Feynman, song dd thj d day bieu didn mdt trgng thai cua khdng thdi gian trong khi dd thj Feynman bilu didn mdt qud trinh tOPng tdc. Ndu md td trgng thdi lopng tO cua vu trg hlnh hpc cua vu trg - thi se ed mdt mgng spin khdng Id vdi khodng 10^^"* d?nh.
Trong LQG, t h i tfch vd didn tich trd thdnh nhOng dgi lopng iopng tO eh? lay nhOng th sd gidn dogn (hlnh 5).
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hgt bdi sg chuyin ddng tOng bode gidn doan cCia cdc ddc trOng trdn mgng.
Tien trien cua hinh hoc theo thoi gian (spin foam)
Ngodi so ehuyin ddng cua hgt vd trodng, hinh hpc cua khdng thdi gian cung thay d i i theo thdi gian. Trong LQG, qud trinh dd bilu d i l n bing nhQng thay d i i eua mgng. Khi thdm thdi gian vdo thay vl
mgng spin ta se cd cdi gpi Id bpt spin (spin foam), cdc dddng ndi biin thdnh nhOng mdt 2 chieu, cdn cdc nut bidn thdnh dOdng 1 chidu (hlnh 6)
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Hinh 5: mdt trong nhOng ket qui trung tam cua LQG gin lidn vdi viec tinh toin the tich vi dien tich. Hay xdt mdt vd hinh ciu xie dinh bien B cua mdt vung khdng gian cd mdt thS tfch nio dd.
Theo vat ly co dien (khdng lUdng td), the tich phii la mdt sd' thUc duang. Song theo LQG thi tdn tai mdt thi tich toi thieu khic khdng (khoing luy thda bic ba cua do dii Planck, hay 10^ cnf), vi diSu niy dan din mdt day gian doan cic tri sd' dd'i vdi cic thi tfch idn han. TUdng tU nhU viy, ton t^i mdt didn tfch cue tiiu (khoing binh phuang dd dii Planck, hay 10^ cnf) vi mdt day giin do^n cic tn so doi vdi nhOng didn tfch Idn han. Phi giin do^n di$n tich (trii), thi tfch (giijfa) trdng tUdng tU nhU phi giin do^n cic mUc ning luang luang tCl cua nguydn tCf hydrogen (phii)
Dua vat chat vao
NhOng mgng spin md td khdng thdi gian. The cdn vat chat vd nang lOdng thi sao? Ldm thd ndo d l bilu didn hgt vd trodng chid'm nhOng vi tn vd nhOng vung trong khdng gian? NhOng hgt nho electron se tOdng Ong vdi nhOng logi nut ed them nhieu ddc trdng (labels) bo sung tgi nut. Cae trodng, vf dg trOdng dien tO se dope bilu didn nhd them nhieu dae trOng hPn trdn ede dOdng ndi cua mgng.
Chung ta bilu didn chuyin dpng cua trOdng vd
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Hinh 6: bpt spin. Theo thdi gian du hinh 1 da biin thidn thinli ciu hlnh 4 qua cic tr^ng thii trung gian 2 vi 3
Trdn hlnh 6 ta ed mdt bpt spin, mpt tid't didn cat ngang bpt spin tgi mdt thdi dilm ndo dd ta cd Igi mot mgng spin. Vd tO cdu hlnh 6/1 dd'n ed'u hinh cu6i cung 6/4 qua cdc cdu hinh trung gian 6/2 vd 6/3 ta thd'y hlnh hpc da thay ddi tO cdu hinh cd 3 t h i tfch vd 6 mdt trd thdnh mdt cau hinh vdi 1 t h i tich va 3 mdt.
LQG va 16 den
Ld den Id k i t qud cua GR (General Relativity - Ly thuyd't tOdng ddi tong qudt) song Id den cung neu Idn nhOng dilm yd'u cua GR vi Id den Id mdt diem ky di (hinh 7) d dd'y dd cong khdng thdi gian va mdt dd vat chdt tidn d i n vd cung. LQG ndi gi vd l5 den?
Trong nhOng nam 70 eua the ky XX, Stephen Hawking vd Jacob Bekenstein da chOng minh Id den cd khd nang bOc xg. Ridng GR khdng t h i suy ra dieu nay dope, d l hiiu dieu nay phdi cd sO vien trp cua cdc hidu Ong lopng tO. Mot vat bOc xg phai cd nhOng tfnh chd't nhidt ddng hpc. Hawking chOng minh rang. Id den ndi chung cd kha nang bOc xa
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Hlnh 7: Id den la mdt diim ky di d dd dd cong tiin din vd cUc, cung tai dd GR dat din nhOng gidi han cua minh: GR khdng cdn dung tai day nOa, viy khdng md ti dUdc diSu gi se xiy ra d day.
Khi nang tien doin dUng lai d "chan trdi", sau chin trdi ta chua biit duac ve khdng - thdi gian
photon, nPthno, graviton gid'ng nho mdt vdt den bj dd't ndng dd'n nhidt dp 9 = hg/(IncK)^ K hang sd Boltzmann.
Entropie vd'n Id dae trong eua nhOng tap hPp cdc trgng thdi vi md cua Id den. Bekenstein vd Hawking da thid't lap mdt edng thOc ndi lien entropie vdi diin tfch Chan trdi S = (1/4)AG, A = didn tfch chan trdi, G = hang so Newton.
Ta ed t h i lay mdt qua bdng cao su d l hiiu dp sud't vT md giO eho qua bdng phdng ldn, ta phai truy nguydn dp sud't vT md dd tO nhOng va chgm eua ede phdn tO gay ndn nhOng dp sud't vi md Idn thdnh qud bdng. Dd'i vdi Id den ta can md td dope nhOng trgng thai vi md. LQG lam dope vide dd. TO sO md td vi md CP ban hdn trong LQG, nam 1996 Carlo Rovelli da tim ra cdng thOe entropie cua Id den va cdc kd't qua cua Stephen Hawking (Cambridge) vd Jacob Bekenstein (Princeton).
BigBounce va BigBang
BigBounce: mdt gia thuye't mdi ve dilm phdt sinh cua vu tru. TO trOde dd'n nay ngOdi ta van eho vg nd
Idn BigBang la cP che lam xud't hidn vu trg. Song gia thid't BigBang phdi mac nhan mdt dilm ky dj vdi mat dp vat chat vd cung Idn, day la diem yd'u cua thuyet BigBang, vl d nhOng dieu kien nho vay thuyet lopng tO phai vdo cudc, do dd the cue se khac di nhieu: ta se thd'y thay vi BigBang se cd BigBounce.
Vu tru cua ehung ta khdng bat dau bang BigBang md bang mdt BigBounce (tid'ng Anh bounce ed nghTa Id sd nay bat): trode BigBounce chung ta cd mdt vu trg dd'i xOng gOPng (tOc la dd'i xOng md ehung ta cd dope khi dOng trode mdt cdi ggpng, eon ngOdi trong gOPng ed qua tim nam bdn phai) vdi vu trg hidn tgi. Vu trg trode BigBounce co nen Igi dd'n mdt kfeh thode nhd, ddn dd'n mdt mat dp coc dgi. Qua trinh CO ndy ed tdn tid'ng Anh la BigCrunch. Qua trinh eo nen nay da kfch ldn mdt qua trinh bung ra, dieu khien bdi cae hidu Ong lupng tO, vd chung ta cd BigBounce (hinh 8).
Hlnh 8: sU niy bit Idn BigBounce cCia vu tnj trdng gid'ng nhU sU niy bit cua mdt qui bdng di khi va ch^m vdi mdt mat tiip xuc nhu mit dit
Ndng lopng td'i vd vdt chd't td'i md ban chd't choa dope biet rd Id 2 didu bf an cua vu trg. Song mdt didu bf an nOa kinh ngge hon Igi Id BigBang. Ddy Id mdt dilm ky dj d dd mpi dgi lOdng tid'n dd'n vd cung.
Tgi dd GR khdng Ong dgng dOdc. Ddy Id thdi dilm
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ma GR sgp do (effondrement de GR). Vdn dd td nhi vd mat sieu hlnh cung nho ve mdt todn hpc BigBang Id hdn da can dOdng trong ljch sO vu trg. BigBang gay nen mdt tinh hudng mdu thudn vdi chfnh ngay GR la ly thuye't da tgo sinh ra nd. Gidi quydt vd'n dd vu trg la phdi gidi quyd't van dd BigBang.
Nam 1999, Martin Bojowald [5] chOng minh rdng, BigBang bien mat vd thay vdo dd la BigBounce.
Theo tinh than eua LQG, tdc gid trdn khdng sO dgng cdc phOdng trinh vi phdn thdng thodng md sO dgng cdc phOPng trinh sai phdn (difference equations), cdc phOPng trinh ndy phdn khdng thdi gian thdnh nhOng khoang cdch gidn dogn. Nhidu ngOdi todng rang, cdc phOPng trinh sai phdn ch? cho Igi cdc kd't qud cua ede phOPng trinh vi phdn d mdt dgng khdc, song nhOng didu mdi Ig da phdt sinh.
Martin Bojowald chOng minh rang, ed'u true hgt eua khdng thdi gian ddn dd'n cdc hidu Ong lOpng tO gdy ndn mdt lOe day qudn binh hap ddn vd tO dd trdnh dope BigBang. LQG da ldm thay doi hlnh dung cua ehung ta dd'i vdi vu trg vd ljch sO tidn triln eua vu tru, md ra cdnh cOa nhin vd phfa trode kia eua BigBang.
Tinh hudng tOdng to nho trong nguyen tO Hydrogen eac hidu Ong lopng tO ngdn khdng eho electron rPi vdo proton. Sau dd, cdc hidu Ong ndy giam di vd ta ed vu trg nhO trong GR. Bdy gid ta thu dope hlnh dnh cua mdt nut eo chai (hinh 9).
Hinh 9: theo LOG thi BigBang nhudng cho cho BigBounce. Khong cdn tdn tai thdi diim ban dau nOa mi thay via dd ii mgt d^ng cd chai. tJlat do d diy rit idn nhung khdng vd cung. Trudc c6 chai vu tru cd mot pha co lai vi sau ci chai ta cd qui trinh gian nd hidn nay
Tgi dd mdt dd rd't Idn song hOu hgn. Vd sO niy bdt (bounce) ndy cd t h i xdy ra nhidu Idn, cd t h i vd ci/c ldn tgo ndn m^t bOc tranh chu ky trong ljch sO vu trg. Thay vl BigBang ta cd BigBounce. Dieu nay gidi quyd't dope vdn dd khdng cdn mdu thuIn todn hpc nOa: ly thuyd't trd thdnh ddu tranh dope dilm ky di ban ddu.
Lam phat
Trong ndm 2010, nhdm cua Ashtekar di xa hdn nOa chOng minh rdng LQG ed t h i gdy ndn Igm phdt.
Trong GR Igm phdt dope dOa vdo b i n g tay, d ddy thi khdng nhO vdy. Chu y rang ndu trong cdc ly thuydt khdc LQG, ngOdi ta phdi doa ad hoe (khdng ddtfnh trddc) hidn topng gian nd Igm phdt (inflation) [5] d l gidi thfch tinh trgng vu trg hidn nay thi LQG gidi thfch dope qud trinh Igm phdt nhO qud thnh ddt sinh luc hap d i n bidn thdnh loc d l y vdo luc sd sinh cua vu trg, nho mdt hd qud td't yd'u eua ed'u true nguydn tCf cua khdng thdi gian.
Nhung het qua ciia LQG
Trong LQG, cdc nhd vat ly da thu dope nhOng kd't qua chfnh sau day: 1 - Todn tO t h i tfch cd cdc tri hdng (eigenvalue) lam thdnh mdt pho gidn dogn va trj coc tiiu cua t h i tfch bang 10"^^ cm^ (lap phOdng cua dp ddi Planck); 2- Cdc th hdng cua todn ttf didn tfch cung ldm thdnh mdt pho gidn dogn vdi tri cOc tiiu bang 10-^^ cm^ (binh phOdng eua dp dai Planck); 3- Thu Igi cdng thOc entropy Id den cue Bekenstein vd ly thuyd't bOc xg Hawking tO quan die'm vi md; 4- John Barrett, Winston Falrbairn (Dgi hoe Nottingham, Anh) chOng minh ring, d kfch thode Idn LQG quy vd GR va d luc trodng hap ddn yeu quy vd hdp ddn Newton; 5- Eugenie Bianchi va You Ding (Dgi hpc Aix-Marseille, Phap) tim Igi dope ldi gidi Friedman md td gian nd trong GR; 6- Mot thanh tOu Idn cua LQG la trong vu tru hpc: LQG da dan ddn BigBounce (thay vl BigBang) tO mdt vu trg tien bdi dang eo Igi. LQG cd nhidu diSm tddng dong
vdi ly thuydt CDT (Causal Dynamical Triangulation) det khdng thdi gian bang nhdng dan hlnh (simplex) thda man didu kiin nhdn qud.
Kiem nghiem LQG
Cae tac gid LQG chOng minh rang, van td'e dnh sang khdng phdi Id hang sd ma phu thudc vao nang
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Su bung no tia gamma (GRB)
Khong thoi gian gian doan
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ria gamma
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Irai dat
Nhieu ty nam anh sang Ve tinh GLAST
Hlnh 10: nhOng bOng ni gamma xiy ra nhiiu ty nim vS trUdc phdng ra nhiSu tia gamma. Theo LQG mdi photon chiim mdt vung cic dudng (nil) t^i mdi thdi diim khi chuyin ddng trdn m^ng spin vd'n li khdng gian. Bin chit giin do^n cCia khdng gian lim cho nhOfng tia gamma ning IU0ng cao chuyin ddng nhanh han cic photon ning iUdng thi'p han. Niu cic tia gamma tCf GRB din theo cac thdi diim khic nhau thi dd se li mdt thing 101 cCia LQG vi ddng thdi li cich kiim nghi$m LQG
lopng cua photon (kd't qud ndy vi phgm ly thuyd't tOdng ddi eua Einstein). Day Id mdt kd't qud quan trpng suy ra tO ed'u true gidn dogn eua khdng gian, kd't qua nay cho phep kilm tra thOe nghidm ly thuyd't LQG (ed t h i nhd trgm tham dd GLAST Gamma ray Large Area Space Telescope Kfnh vidn vpng tia gamma vu tru thj trodng rdng). LQG da vi phgm didu thidng lidng ve td'e dp dnh sdng. Anh sdng nd'u cd dp ddi sdng Idn hdn nhidu khodng cdch giOa ede "nguydn tO" cua khdng gian thi khdng chju anh hodng gi nhO Id dnh sdng "khdng thd'y" dOde ed'u true eua khdng gian vd td'e dp dnh sdng bang td'e dp dnh sdng trong chan khdng. Ngopc Igi, nd'u dp dai sdng cung ed vdi khoang each giOa ede "nguyen tO" khdng gian hoac nho hdn thi dnh sang chju dnh hodng cua tfnh gidn dogn cua khdng gian vd tdc dp phu thudc vao dp ddi sdng. Nho the, mdt photon nang lopng eao ed dd ddi sdng nhd cd Planck ed t h i chju anh hodng cua ed'u true lOdng tO cua khdng gian, dd Id kd't qua cCia LQG.
Ngodi ta sO dgng cae bung n l tia gamma (sursauts gamma) d l nghien cOu hidn tOdng ndy:
nho the nd'u cd mdt sO phan bd trong qud trinh di tdi mat dd't cua cdc photon vdi nhieu bOde sdng khdc nhau, thi dd se la mdt dilm thang Ipi quan trpng eua LQG (hlnh 10).
Ket luan
Ly thuye't LQG hidn nay Id mdt ly thuye't cd nhieu triln vpng trong vd'n dd lOpng tO hda hd'p ddn. LQG
sO dgng nhidu edng cg todn hpc phOe tgp, song ddy cung Id tinh hudng ehung eua mpi ly thuydt mdi nhO luc GR ra ddi. Mdt dieu quan trpng Id LQG dOa ra dope mdt thf nghidm d l kilm chOng nd, dd Id vd'n dd tdc dp truyen cua dnh sdng trong khdng thdi gian gidn dogn phg thude vdo ndng lOpng photon •
Biin dich: Cao Chi
Tai lieu tham khao va chii thich
[1] Bernard Romney, La gravite quantique a boucies, La Reclierche thang 12.2011, N°458.
[2] Aur6lien Barrau and Francesca Vidotto, De I'autre cdte du BIgbang, La recherche thdng 12.2011, N° 458.
[3] Lee Smolin, Atoms of Space and Time, Scientific American, s6 thdng 1.2004.
[4] Carlo Rovelli, Loop Quantum Gravity, http://www.pitt.edu/
-rovelli
[5] Martin Bojowald, BigBang or BigBounce? New Theory on the Universe's Birth, Scientific Amerrican s6'thang 10.2008. BigBounce c6 nghTa Id sU niy b$t Idn nhu khi qud bong da ch^m manh vao cac c6t khung thdnh.
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